| Missense mutations | Often result in partially functional enzymes |
| Nonsense/frameshift mutations | Lead to truncated non-functional proteins |
| Splice site mutations | Cause exon skipping |
| Large deletions | Less common[@matsuo2020] |
| Overinhibition | Excessive glycine binding to GlyRs in brainstem and spinal cord causes profound hypotonia, apnea, and respiratory failure[@kurki2019] |
| Brainstem | Respiratory centers, leading to apnea and respiratory failure |
| Cerebellum | Purkinje cell loss, contributing to ataxia |
| Corpus callosum | Dysmyelination and agenesis |
| Elevated CSF glycine | Typically 10-30 times normal levels (CSF:plasma ratio >0.08 is diagnostic) |
| Elevated plasma glycine | 3-10 times normal |
| Absence of ketoacidosis | Distinguishes NKH from ketotic hyperglycinemias |
| Elevated urine glycine | Secondary to increased plasma levels[@schiffmann2019] |
| Databases | OMIMOrphanetClinicalTrialsPubMed |