The NM_005629.4:c.820G>A variant in SLC6A8 is a missense variant predicted to cause the substitution of a valine by a methionine at amino acid position 274 (p.Val274Met). This variant has been previously reported in one individual with hypotonia, global developmental delay, and microcephaly, but who was found to have normal urine creatine and guanidinoacetate levels on two separate occasions and a de novo variant in KAT6A as an alternate molecular cause of his symptoms (PMID 27133397) (BS2). In gnomAD v2.1.1, the highest population allele frequency is 0.00015 (14/91767 alleles) in the European (Non-Finnish) population and there are 4 hemizygotes across all populations, which is greater than the ClinGen CCDS VCEP’s threshold for PM2_Supporting (<0.00002) but less than the ClinGen CCDS VCEP’s threshold for BS1 (>0.0002), such that neither of these criteria are met. The computational predictor REVEL gives a score of 0.559, which is less than the ClinGen CCDS VCEP’s threshold for PP3 (>0.75) but greater than the ClinGen CCDS VCEP’s threshold for BP4 (<0.2), such that neither of these criteria are met. There is a ClinVar entry for this variant (Variation ID: 4416002). In summary, this variant meets the criteria to be classified as a variant of uncertain significance for creatine transporter deficiency. SLC6A8-specific ACMG-AMP criteria applied, as specified by the ClinGen CCDS VCEP (Specifications Version 1.1.0): BS2. (Classification approved by the ClinGen Cerebral Creatine Deficiency Syndromes Variant Curation Expert Panel on November 8, 2024)