The NM_005629.4: c.1271G>A variant in the SLC6A8 is a missense variant in exon 9 predicted to result in substitution of glycine by aspartic acid at position 424 (p.Gly424Asp). This variant has been reported in three male siblings with mild intellectual disability, elevated urine creatine/creatinine, and reduced creatine peak on brain MRS (PP4_strong). Their mother was heterozygous for the same variant, had a mild reduction of creatine on MRS and indicate the mother had attention deficit, impulsiveness and intelligence below average (PMID: 24137762) (PP1_moderate). This variant was not found in gnomAD v2.1.1. (PM2_supporting). In silico predictions suggest this variant will be damaging to the function of the canonical protein (REVEL = 0.90) (PP3). There is no ClinVar entry for this variant. Functional studies of this variant demonstrated residual creatine uptake of 29% of the wildtype transfection level, indicating that this variant may affect the function of the protein but not meeting the threshold required by the ClinGen Cerebral Creatine Deficiency Syndromes VCEP (<10%) (PMID: 22281021) (PS3_Supporting not met). In summary, this variant meets criteria to be classified as a likely pathogenic variant for creatine transporter deficiency. SLC6A8-specific criteria applied, as specified by the ClinGen CCDS VCEP (Specifications Version 1.1.0) (PP4_strong, PP1_moderate, PM2_supporting, PP3). (Classification approved by the ClinGen Cerebral Cretine Deficiency Syndromes Variant Curation Expert Panel on March 29, 2024).