The NM_005629.4:c.912G>C in SLC6A8 is a missense variant predicted to cause substitution of glutamine by histidine at amino acid 304 (p.Gln304His). The variant is absent in gnomAD v2.1.1 (PM2_Supporting). The computational predictor REVEL gives a score of 0.749 (>0.75 provides evidence that correlates with impact to SLC6A8 function). The computational splicing predictors SpliceAI and varSEAK predict that the variant may disrupt the donor splice site of intron 5 (PP3). Another variant at this position, c.912G>A (p.Gln304=), which has been reported in a patient with creatine transporter deficiency and may disrupt normal splicing of SLC6A8 (PMIDs: 17101918, 23660394, 29478817) has been classified as likely pathogenic by the ClinGen CCDS VCEP. Comparison of the in silico predictions for c.912G>A (p.Gln304=) and c.912G>C (p.Gln304His) suggest that c.912G>A (p.Gln304=) is less damaging to splicing than c.912G>C (p.Gln304His) (PS1_Supporting). To our knowledge, c.912G>C (p.Gln304His) has not been reported in the literature and results of functional and splicing studies are unavailable. However, there is a ClinVar entry for this variant (Variation ID: 420991); follow up with the lab did not provide sufficient evidence to apply additional criteria. In summary, this variant meets the criteria to be classified as a variant of uncertain significance for X-linked creatine transporter deficiency. SLC6A8-specific ACMG/AMP criteria applied, as specified by the ClinGen CCDS VCEP (Specifications Version 1.1.0): PP3, PM2_Supporting, PS1_Supporting. (Classification approved by the ClinGen CCDS VCEP, Jan 12, 2023)