The NM_000156.6:c.316C>T variant in GAMT is a frameshift variant predicted to cause a premature stop codon in biologically relevant exon 2/6 leading to nonsense mediated decay in a gene in which loss-of-function is an established disease mechanism (PVS1). Two patients with this variant have been reported with clinical features consistent with GAMT deficiency, elevated guanidinoacetate in urine or plasma, and low or absent creatine peak on brain magnetic resonance spectroscopy (MRS), and one of these patients also had evidence of a guanidinoacetate peak on brain MRS (PMID: 20049533, 24415674, 29506905) (PP4_Strong). Both patients (who have different descriptions) are compound heterozygous for the variant and c.407C>T (p.Thr136Met), phase unknown (PMID 20049533, 24415674, 29506905). The allelic data from these patients will be used in the assessment of p.Thr136Met and is not included here to avoid circular logic. This variant is absent in gnomAD v2.1.1 (PM2_Supporting). There is a ClinVar entry for this variant (Variation ID: 566624). In summary, this variant meets the criteria to be classified as pathogenic for GAMT deficiency based on the ACMG/AMP criteria applied, as specified by the ClinGen Cerebral Creatine Deficiency Syndromes Variant Curation Expert Panel (Specifications Version 1.1.0): PVS1, PP4_strong, PM2_supporting. (Classification approved by the ClinGen CCDS VCEP on June 6, 2022).