The NM_005629.4(SLC6A8):c.1171C>T (p.Arg391Trp) variant in SLC6A8 is a missense variant predicted to cause substitution of Tryptophan for Arginine at amino acid 391 (p.Arg391Trp). There is a ClinVar entry for this variant (Variation ID:465141, Conflicting Interpretations) with classifications as Pathogenic (n=2) and as a Variant of Uncertain Significance (n=1). This variant is absent from gnomAD v2.1.1, therefore PM2_Supporting criteria is applicable. The computational predictor REVEL gives a score of 0.078 which is below the threshold of 0.20, suggesting a benign effect of this variant on protein function, therefore BP4 criteria is applicable. The p.Arg391Trp variant has been reported in a male with intellectual disability, speech delay, and seizures who had biochemical studies including elevated Creatine:Creatinine ratio in urine and MRS demonstrating reduced Creatine peak in brain (Patient 2; [PMID:21910234]), which meets criteria for PP4_Strong. The patient from Mencarelli, 2011 [PMID:21910234] was also found to have the c.1171C>T (p.Arg391Trp) variant de novo, meeting criteria for PS2. Site directed mutagenesis and creatine uptake studies were performed with the p.Arg391Trp variant, and these studies demonstrated creatine uptake <10% wildtype cells (<150 μM creatine used), meeting PS3_Supporting criteria [PMID:30885608]. This variant has also been identified in a male with intellectual disability and/or seizures but without biochemical evidence of creatine transporter deficiency (elevated urine creatine:creatine, MRS demonstrating reduced creatine peak) [PMID:16738945], and given the absence of biochemical evidence of Creatine Transporter Deficiency in this individual evidence for pathogenicity was not scored. In summary, this variant meets the criteria to be classified as Likely Pathogenic for Creatine Transporter 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): PP4_Strong, PS2, PS3_Supporting, PM2_Supporting, BP4. Classification changed from VUS to LP given the only benign/conflicting evidence for this variant is in silico evidence. Using the new point scoring system from Tavtigian et.al 2020 [PMID:32720330], this variant meets criteria for Likely Pathogenic (Score: 4+4+1+1-1 = 9 points; Likely Pathogenic). (Classification approved by the CCDS VCEP Nov 10, 2022)