CHRNB2 Protein — Nicotinic Acetylcholine Receptor Beta 2 Subunit
Overview
CHRNB2 encodes the beta-2 (β2) subunit of neuronal nicotinic acetylcholine receptors (nAChRs), which are pentameric ligand-gated ion channels critical for fast cholinergic synaptic transmission in the central and peripheral nervous systems. The β2 subunit is a defining component of the most abundant nicotinic receptor subtype in the mammalian brain—the α4β2* receptor—where it plays essential roles in cognitive processes, attention, learning, memory, reward signaling, and motor control. Dysregulation of CHRNB2-containing receptors has been strongly implicated in the pathogenesis of Alzheimer's disease (AD), Parkinson's disease (PD), epilepsy, nicotine addiction, and various neuropsychiatric disorders[@gotti2006][@lloyd2006].
The neuronal nAChRs belong to the Cys-loop receptor superfamily, which also includes GABA_A, glycine, and 5-HT3 receptors. Unlike muscle-type nAChRs found at the neuromuscular junction, neuronal nAChRs containing the β2 subunit are primarily expressed in the brain and autonomic ganglia, where they modulate neurotransmission and neuronal excitability. The α4β2 receptor (\"\\" denotes possible inclusion of other subunits) is the predominant subtype in the hippocampus, cortex, thalamus, and basal ganglia—brain regions critical for cognition, movement, and reward processing[@han2007].
<div class="infobox infobox-protein">
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<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">CHRNB2 Protein</th></tr>
<tr><td><strong>Protein Name</strong></td><td>nAChR β2 Subunit</td></tr>
<tr><td><strong>Gene Symbol</strong></td><td>CHRNB2</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[P17787](https://www.uniprot.org/uniprot/P17787)</td></tr>
<tr><td><strong>PDB IDs</strong></td><td>5KXI, 6CNG, 7K7Y</td></tr>
<tr><td><strong>Chromosome</strong></td><td>1q21.3</td></tr>
<tr><td><strong>NCBI Gene ID</strong></td><td>[1144](https://www.ncbi.nlm.nih.gov/gene/1144)</td></tr>
<tr><td><strong>Protein Family</strong></td><td>Cys-loop receptor family</td></tr>
<tr><td><strong>Structure</strong></td><td>Pentameric ligand-gated ion channel</td></tr>
<tr><td><strong>Subcellular Location</strong></td><td>Plasma membrane</td></tr>
<tr><td><strong>Molecular Weight</strong></td><td>~57 kDa (glycosylated)</td></tr>
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<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>
</div>
Molecular Biology and Structure
Gene Organization and Expression
The CHRNB2 gene is located on chromosome 1q21.3 and spans approximately 6.5 kb. It consists of 6 exons encoding a protein of 493 amino acids. CHRNB2 is expressed predominantly in the central nervous system, with highest levels in:
- Hippocampus: CA1-CA3 regions and dentate gyrus
- Cerebral cortex: Layer I-VI, particularly layer IV
- Thalamus: Relay nuclei and intralaminar nuclei
- Basal ganglia: Striatum, nucleus accumbens, substantia nigra pars compacta
- Brainstem: Pedunculopontine nucleus, laterodorsal tegmental nucleus
- Autonomic ganglia: Superior cervical ganglion, nodose ganglion
The developmental expression pattern shows that CHRNB2 subunit expression increases during postnatal development, peaking in early adulthood and declining with aging—paralleling the trajectory of cognitive function[@han2007][@taly2015].
Receptor Architecture
Neuronal nAChRs are pentameric assemblies composed of five subunits arranged around a central ion channel pore. Each subunit contains:
N-terminal extracellular domain (~210 residues): Contains the acetylcholine binding site at subunit interfaces
Transmembrane domain (~140 residues): Four α-helices (M1-M4), with M2 forming the ion channel pore
Intracellular loop (~60 residues): Between M3 and M4, contains trafficking signals
C-terminal extracellular domain (~20 residues): Short cytoplasmic tailThe β2 subunit contributes to the receptor's pharmacological profile and determines channel properties such as conductance, desensitization kinetics, and calcium permeability. When combined with α4 subunits, the resulting α4β2 receptor exhibits:
| Property | Value |
|----------|-------|
| Single-channel conductance | 30-40 pS |
| Mean open time | 1-5 ms |
| Desensitization time constant | 100 ms to several seconds |
| Calcium permeability | 1.5-5% of total current |
| Reversal potential | ~0 mV (Na+ equilibrium) |
Subunit Composition and Diversity
The β2 subunit can combine with various α subunits to form functionally distinct receptors:
| Receptor Subtype | Stoichiometry | Brain Region | Key Functions |
|------------------|---------------|---------------|---------------|
| α4β2* (predominant) | (α4)₂(β2)₃ | Hippocampus, cortex, thalamus | Cognitive processes, attention |
| α3β2 | (α3)₂(β2)₃ | Autonomic ganglia | Peripheral cholinergic signaling |
| α2β2 | (α2)₂(β2)₃ | Hippocampus, cortex (lower expression) | Similar to α4β2 |
| α5-β2 | (α4)₂(β2)₂α5 | Cortex, hippocampus | Enhanced calcium permeability |
| β2-δ/ε | (β2)₃(δ or ε) | Neuromuscular junction (mutant) | Not brain-localized |
The asterisks (*) in nomenclature indicates that additional subunits (such as α5 or β3) may be incorporated into the receptor complex, altering its pharmacological and physiological properties.
Structural Insights from Cryo-EM
Recent cryo-electron microscopy studies have provided atomic-resolution structures of α4β2 nAChRs:
- 5KXI: Human α4β2 receptor in apo state
- 6CNG: Human α4β2 with agonist bound
- 7K7Y: Full receptor in multiple conformational states
These structures reveal the conformational changes underlying channel activation and desensitization, providing templates for structure-based drug design. The agonist-binding site is located at the extracellular interface between α4 and β2 subunits, with key residues determining agonist potency and efficacy.
Normal Physiological Functions
Cholinergic Neurotransmission
CHRNB2-containing nAChRs mediate fast excitatory neurotransmission at cholinergic synapses throughout the brain. Key functions include:
Presynaptic modulation: nAChRs on presynaptic terminals regulate release of glutamate, GABA, dopamine, norepinephrine, and acetylcholine itself
Postsynaptic excitation: Postsynaptic nAChRs mediate fast depolarizing responses to acetylcholine
Neuromodulation: Through presynaptic effects, β2-containing receptors modulate the activity of multiple transmitter systemsCognitive Processes
The α4β2* receptor is critically involved in higher cognitive functions:
- Attention: Nicotinic agonists enhance attentional processing; β2 subunit is essential for this effect
- Learning and memory: Hippocampal α4β2 receptors contribute to spatial memory consolidation
- Working memory: Prefrontal cortex nAChRs support working memory operations
- Executive function: Frontal cortex receptors contribute to cognitive flexibility
Reward and Addiction
The mesolimbic dopamine pathway is modulated by β2-containing receptors:
- Ventral tegmental area (VTA): α4β2 receptors on VTA neurons regulate dopamine release to nucleus accumbens
- Nicotine addiction: β2 subunit is required for nicotine self-administration in rodents
- Genetic variants: CHRNB2 polymorphisms are associated with nicotine dependence susceptibility
Motor Control
In basal ganglia circuits, β2-containing receptors influence motor function:
- Striatal modulation: nAChRs on striatal interneurons and terminals regulate movement
- Substantia nigra: Presynaptic receptors on dopaminergic neurons modulate nigrostriatal signaling
- Motor learning: Sensorimotor integration involves β2 receptor signaling
Role in Alzheimer's Disease
Cholinergic Hypothesis and Receptor Loss
The cholinergic hypothesis of AD posits that loss of basal forebrain cholinergic neurons and subsequent decline in acetylcholine signaling contributes to cognitive deficits. CHRNB2-containing receptors are primary targets of this degeneration:
- Receptor density reduction: Post-mortem studies show 40-70% reduction in α4β2 binding in AD cortex and hippocampus
- Correlation with cognitive decline: α4β2 receptor loss correlates with impaired memory performance
- Selective vulnerability: β2-containing receptors are more vulnerable than α7 receptors
Amyloid-Beta Interactions
Amyloid-beta (Aβ) peptides interact directly and indirectly with α4β2 receptors:
- Direct binding: Aβ1-42 binds to nAChRs with low micromolar affinity, potentially functioning as an antagonist
- Receptor internalization: Aβ promotes internalization of α4β2 receptors
- Calcium dysregulation: Aβ-induced calcium dysregulation is exacerbated through nAChR-mediated calcium influx
- Synaptic dysfunction: Aβ-induced synaptic deficits are partially mediated through nAChR modulation
The relationship between Aβ and nAChRs is complex—while acute activation may be protective, chronic Aβ exposure leads to receptor dysfunction and downregulation[@dineley2020][@yang2017].
Therapeutic Implications for AD
Targeting α4β2 receptors remains a therapeutic strategy for AD:
| Approach | Agent | Status | Mechanism |
|----------|-------|--------|----------|
| AChE inhibitors | Donepezil, Rivastigmine | Approved | Increase synaptic ACh to activate receptors |
| Direct agonists | TC-1734 | Phase II | Selective α4β2 activation |
| Positive allosteric modulators | NS-1738 | Preclinical | Enhance agonist efficacy |
| Antagonists | Mecamylamine | Phase II | Prevent receptor desensitization |
Nicotine and nicotinic agonists have demonstrated cognitive benefits in AD patients, though side effects limit clinical utility. Newer approaches focus on partial agonists with improved safety profiles and allosteric modulators that preserve temporal signaling patterns[@newhouse2012][@peters2013][@usluer2024].
Prodromal and Early Stage Changes
Recent research reveals that nAChR dysfunction begins in prodromal AD:
- MCI subjects: Reduced α4β2 binding in cortex and hippocampus
- Biomarkers: CSF nAChR levels may serve as early biomarkers
- Intervention window: Early intervention may prevent downstream degeneration
Role in Parkinson's Disease
Nigrostriatal Degeneration and Receptor Loss
In PD, the loss of dopaminergic neurons in substantia nigra pars compacta leads to secondary changes in nAChR expression:
- Striatal receptor changes: α4β2 binding is reduced in PD striatum
- Substantia nigra: Loss of nAChR-positive dopaminergic neurons
- Compensatory mechanisms: Early stages may show receptor upregulation
Nicotine as a Neuroprotective Agent
Epidemiological studies suggest that nicotine consumption (via tobacco) reduces PD risk:
- Inverse correlation: Smoking history correlates with reduced PD incidence
- Neuroprotective mechanisms: nAChR activation may protect dopaminergic neurons
- Anti-inflammatory effects: Cholinergic anti-inflammatory pathway via α7 and β2 receptors
Motor and Non-Motor Symptoms
CHRNB2-containing receptors influence both motor and non-motor PD symptoms:
- Motor complications: Levodopa-induced dyskinesias involve nAChR dysfunction
- Cognitive impairment: PD dementia involves nAChR loss similar to AD
- Autonomic dysfunction: Peripheral β2 receptors affect autonomic regulation
Therapeutic Approaches for PD
| Strategy | Agent | Rationale |
|----------|-------|----------|
| Nicotine replacement | Patch, gum | Neuroprotection, motor symptom improvement |
| α4β2 agonists | ABT-089, TC-1734 | Cognitive and motor benefits |
| Combination therapy | Nicotine + dopaminergic drugs | Enhanced efficacy |
| Allosteric modulators | Novel compounds | Improved safety profile |
Clinical trials of nicotine in PD have shown mixed results, with some benefit for motor symptoms but limited overall efficacy. Novel selective agonists and positive allosteric modulators are in development[@bordia2015][@hernandez2022].
Role in Epilepsy
CHRNB2 Mutations and ADNFLE
Autosomal Dominant Nocturnal Frontal Lobe Epilepsy (ADNFLE) was the first human disease linked to neuronal nAChR mutations. CHRNB2 mutations identified in ADNFLE include:
- V287M: First identified mutation, causes gain-of-function
- V287L: Similar gain-of-function phenotype
- I296M: Increased channel activity
- S299L: Enhanced agonist sensitivity
- L301V: Altered desensitization kinetics
These mutations cause increased channel activity in response to acetylcholine, leading to neuronal hyperexcitability, particularly during sleep when cholinergic tone is high. The seizures occur predominantly during non-REM sleep, reflecting the physiological activation of cholinergic neurons during this state[@steinlein2014][@barlow2020].
Mechanisms of Epileptogenesis
Gain-of-function CHRNB2 mutations lead to epilepsy through:
Enhanced excitatory drive: Increased nAChR currents depolarize neurons
Network hypersynchrony: Excessive cholinergic signaling promotes seizure-like activity
Cortical excitability: Altered thalamocortical circuit function
GABAergic modulation: Secondary effects on inhibitory circuitsTherapeutic Implications
CHRNB2-related epilepsy has been treated with:
- Carbamazepine: Sodium channel blocker, first-line therapy
- Topiramate: Multiple mechanisms including nAChR antagonism
- Zonisamide: Broader efficacy in ADNFLE
- Nicotinic antagonists: Mecamylamine (experimental)
Understanding CHRNB2 mutations has provided insights into the role of cholinergic signaling in seizure generation and potential therapeutic targets.
Role in Neuroinflammation
Cholinergic Anti-Inflammatory Pathway
The cholinergic anti-inflammatory pathway represents a neuroimmune regulatory mechanism:
- Vagus nerve: Acetylcholine release inhibits peripheral cytokine production
- α7 nAChR: Primary mediator in macrophages, but β2 receptors also contribute
- Microglial modulation: nAChR activation alters microglial morphology and function
- TNF-α regulation: Nicotinic signaling reduces pro-inflammatory cytokine release
Implications for Neurodegeneration
Chronic neuroinflammation is a hallmark of AD, PD, and ALS:
- Microglial activation: β2 receptors modulate microglial phenotype
- Cytokine production: nAChR signaling reduces inflammatory mediators
- Therapeutic potential: Nicotinic agonists may reduce neuroinflammation
Genetic Variants and Neuropsychiatric Disorders
CHRNB2 Polymorphisms
Genome-wide association studies (GWAS) have identified CHRNB2 variants associated with:
- Nicotine dependence: Multiple SNPs correlate with smoking behavior
- Schizophrenia: Some variants increase risk
- Attention-deficit/hyperactivity disorder (ADHD): Association with impulsivity
- Major depressive disorder: Risk variants identified in some populations
- Cognitive performance: SNP effects on memory and processing speed
Copy Number Variants
Rare CHRNB2 copy number variants have been reported in:
- Intellectual disability: Deletions encompassing CHRNB2
- Autism spectrum disorders: Some patients carry rare variants
- Epilepsy: Duplications may increase seizure risk
Genotype-Phenotype Correlations
| Variant Type | Phenotype | Mechanism |
|--------------|-----------|-----------|
| Missense (gain-of-function) | ADNFLE | Increased channel activity |
| Missense (loss-of-function) | Cognitive deficits | Reduced signaling |
| Promoter variants | Nicotine dependence | Altered expression |
| Rare deletions | Neurodevelopmental disorders | Haploinsufficiency |
Therapeutic Targeting
Approved Drugs Targeting nAChRs
| Drug | Target | Indication | Mechanism |
|------|--------|-----------|-----------|
| Nicotine | α4β2, α3β4 | Smoking cessation | Partial agonist |
| Varenicline | α4β2, α3β2 | Smoking cessation | Partial agonist |
| Cytisine | α4β2, α3β4 | Smoking cessation (Europe) | Partial agonist |
| Donepezil | AChE → α4β2 | AD | Indirect activation |
| Rivastigmine | AChE → α4β2 | AD, PDD | Indirect activation |
| Galantamine | AChE → α4β2 | AD | Indirect activation + PAM |
Drugs in Development
| Agent | Company | Stage | Target |
|-------|---------|-------|--------|
| TC-1734 | Targacept | Phase II | α4β2 agonist |
| AZD-0327 | AstraZeneca | Preclinical | α4β2 agonist |
| NS-1738 | Unknown | Preclinical | α4β2 PAM |
| ABT-594 | Abbott | Discontinued | α4β2 agonist |
Side Effects and Challenges
Nicotinic agonist development faces challenges:
- Cardiovascular effects: Tachycardia, hypertension
- Gastrointestinal: Nausea, vomiting
- Central effects: Insomnia, vivid dreams, headache
- Seizure risk: Particularly in susceptible individuals
- Addiction potential: Especially with high-efficacy agonists
Partial agonists (varenicline, cytisine) offer improved safety profiles with reduced side effects.
Animal Models
Knockout Mice
CHRNB2 knockout mice (Chrnb2⁻/⁻) have been instrumental in understanding receptor function:
- Cognitive deficits: Impaired working memory and attention
- Reduced nicotine response: No nicotine-induced dopamine release
- Altered synaptic plasticity: Impaired LTP in hippocampus
- Protection from nicotine addiction: No nicotine self-administration
- Neurochemical changes: Reduced striatal dopamine, altered GABA signaling
Transgenic Models
Conditional and inducible transgenic models allow:
- Region-specific rescue: Restoration of β2 in specific brain regions
- Temporal control: Inducible expression to study developmental effects
- Mutation modeling: Human disease mutations in mice
Phenotype Summary
| Model | Key Phenotype | Research Utility |
|-------|---------------|------------------|
| Chrnb2⁻/⁻ | Cognitive deficits, no nicotine response | Receptor function |
| Chrnb2⁺/⁻ | Partial deficits, enhanced vulnerability | Haploinsufficiency |
| Humanized | Human CHRNB2 expression | Drug testing |
| ADNFLE mutant | Seizure phenotype | Epilepsy mechanisms |
Biomarkers and Diagnostic Applications
PET Imaging
Radioligands for imaging β2-containing receptors:
- ¹²³I-iodinated ligands: SPECT imaging in research
- ¹⁸F-labeled ligands: PET imaging, limited availability
- ²⁸S-labeled tropanes: High-affinity imaging agents
CSF Biomarkers
Emerging biomarkers include:
- nAChR levels: Reduced in AD and PD CSF
- Soluble receptor fragments: Potential disease markers
- Autoantibodies: Anti-nAChR antibodies in some disorders
Genetic Testing
Clinical genetic testing for CHRNB2:
- ADNFLE diagnosis: Confirmatory testing for epilepsy
- Carrier testing: Family member risk assessment
- Pharmacogenomics: Predicting nicotine dependence risk
Research Directions and Future Perspectives
Novel Therapeutic Strategies
Subunit-selective agonists: Targeting specific receptor configurations
Allosteric modulators: Positive and negative modulators
Signal compartmentation: Preserving physiological signaling patterns
Combination therapies: nAChR agonists with other mechanismsBiomarker Development
- Early detection: Identifying at-risk individuals
- Treatment response: Predicting therapeutic efficacy
- Disease progression: Monitoring neurodegeneration
Gene Therapy Approaches
- Viral vector delivery: Direct CNS delivery of CHRNB2
- CRPR editing: Correcting pathogenic mutations
- Regulated expression: Inducible systems for controlled therapy
Key Research Findings
1. Discovery of ADNFLE Mutations
Steinlein and colleagues identified the first CHRNB2 mutation (V287M) causing autosomal dominant nocturnal frontal lobe epilepsy, establishing a direct link between neuronal nAChR dysfunction and human epilepsy. This discovery revolutionized understanding of genetic epilepsy mechanisms.
2. Cryo-EM Structure Determination
Morales-Perez and colleagues solved the first atomic-resolution structure of the human α4β2 nAChR, providing unprecedented insights into the structural basis for agonist binding, channel gating, and allosteric modulation.
3. Role in Nicotine Addiction
The critical role of β2-containing receptors in nicotine reward was demonstrated through knockout mice studies, showing that CHRNB2 deletion eliminates nicotine self-administration and dopamine release in the nucleus accumbens.
4. Cognitive Enhancement in AD
Multiple clinical trials have demonstrated that nicotinic agonists can improve cognitive function in AD patients, validating α4β2 receptors as therapeutic targets for dementia.
Related Pages
- [CHRNB2 Gene](/genes/chrnb2)
- [Nicotinic Acetylcholine Receptors](/entities/nicotinic-receptors)
- [Cholinergic Signaling in Neurodegeneration](/mechanisms/cholinergic-signaling-neurodegeneration)
- [Alzheimer's Disease](/diseases/alzheimers-disease)
- [Parkinson's Disease](/diseases/parkinsons-disease)
- [Epilepsy](/diseases/epilepsy)
- [Acetylcholinesterase Inhibitors](/therapeutics/acetylcholinesterase-inhibitors)
External Links
- [UniProt: CHRNB2 - P17787](https://www.uniprot.org/uniprot/P17787)
- [NCBI Gene: CHRNB2](https://www.ncbi.nlm.nih.gov/gene/1144)
- [Ensembl: CHRNB2](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000137336)
- [PDB: 5KXI (α4β2 nAChR)](https://www.rcsb.org/structure/5KXI)
- [IUPHAR: nAChR Subunits](https://www.guidetopharmacology.org/GRAC/FamilyIntroductionForward?familyId=8)
- [GeneCards: CHRNB2](https://www.genecards.org/cgi-bin/carddisp.pl?gene=CHRNB2)
- [OMIM: CHRNB2](https://www.omim.org/entry/118507)
References
[Han et al., Distribution of nicotinic receptors in the human brain and clinical relevance (2007)](https://pubmed.ncbi.nlm.nih.gov/17207768/)
[Gotti et al., Nicotinic acetylcholine receptors in the brain: from circuitry to function (2006)](https://pubmed.ncbi.nlm.nih.gov/16771899/)
[Lloyd & Williams, Neuronal nicotinic receptors as drug targets (2006)](https://pubmed.ncbi.nlm.nih.gov/16678550/)
[Newhouse et al., Nicotinic acetylcholine receptor agonists for neuroprotection (2012)](https://pubmed.ncbi.nlm.nih.gov/22639998/)
[Peters et al., Nicotinic receptors in aging and dementia (2013)](https://pubmed.ncbi.nlm.nih.gov/23571419/)
[Bordia et al., Nicotinic receptors as targets for Parkinson's disease therapy (2015)](https://pubmed.ncbi.nlm.nih.gov/25576122/)
[Lombardo & Maskos, Alpha4beta2 nicotinic receptors in CNS disorders (2018)](https://pubmed.ncbi.nlm.nih.gov/30649666/)
[Yang et al., Role of nicotinic acetylcholine receptors in Alzheimer's disease (2017)](https://pubmed.ncbi.nlm.nih.gov/27569905/)
[Maurice et al., Neuronal nicotinic acetylcholine receptor dysfunction in prodromal Alzheimer's disease (2021)](https://pubmed.ncbi.nlm.nih.gov/34039357/)
[Barlow et al., Beta2 subunit-containing nicotinic receptors in the pathogenesis of epilepsy (2020)](https://pubmed.ncbi.nlm.nih.gov/31759022/)
[Dineley et al., Beta-amyloid peptide and nicotinic acetylcholine receptors (2020)](https://pubmed.ncbi.nlm.nih.gov/32816084/)
[Hernandez et al., Nicotinic receptor modulation for neuroprotection in Parkinson's disease (2022)](https://pubmed.ncbi.nlm.nih.gov/35051416/)
[Yang et al., Alpha4beta2 nAChR agonists for cognitive enhancement in neurodegeneration (2022)](https://pubmed.ncbi.nlm.nih.gov/34742981/)
[Kawasaki et al., Neuronal nicotinic receptors in cognitive decline: new therapeutic strategies (2024)](https://pubmed.ncbi.nlm.nih.gov/38097342/)
[Usluer et al., α4β2 nicotinic acetylcholine receptor agonists in Alzheimer's disease trials (2024)](https://pubmed.ncbi.nlm.nih.gov/38647328/)
[Zhang et al., CHRNB2 variants associated with neuropsychiatric disorders (2023)](https://pubmed.ncbi.nlm.nih.gov/36539652/)
[Steinlein, Genetic disorders causing epilepsy: nicotinic acetylcholine receptor mutations (2014)](https://pubmed.ncbi.nlm.nih.gov/25144149/)
[Xu et al., Nicotinic acetylcholine receptors and microglia in neurodegeneration (2019)](https://pubmed.ncbi.nlm.nih.gov/31213987/)
[Zappettini et al., Cholinergic anti-inflammatory pathway and nicotinic receptors (2022)](https://pubmed.ncbi.nlm.nih.gov/35030486/)
[Taly et al., Neuronal nicotinic acetylcholine receptors: structure, function and pathology (2015)](https://pubmed.ncbi.nlm.nih.gov/25753741/)