The NM_005629.4:c.1169C>T variant in SLC6A8 is a missense variant that is predicted to result in the substitution of proline by leucine at amino acid 390 (p.Pro390Leu). The variant was shown to segregate in a family from a heterozygous mother with mild intellectual disability, to two affected sons and an affected daughter. A clinically unaffected daughter did not carry the variant (PMID: 28065824) (PP1_Strong, PP4_Strong). In addition, two unrelated males with intellectual disability, and an affected brother of one of these individuals, are hemizygous for the variant (PMID: 15154114, 21267006. Fibroblasts from a patient with this variant were shown to be "deficient in creatine uptake" (PMID: 17465020), but further details were not provided, other than the patient is unrelated to the patient reported in PMID: 15154114. The variant is absent in gnomAD v2.1.1. (PM2_Supporting). When transiently expressed in SLC6A8-deficient fibroblasts, the variant resulted in reduced creatine transport. However, the study does not meet the specifications of the CCDS VCEP (must be done with <125 mM creatine while the study used 500 uM). The computational predictor REVEL gives a score of 0.942 which is above the threshold of 0.75, evidence that correlates with impact to SLC6A8 function (PP3). In summary, this variant meets the criteria to be classified as pathogenic for creatine transporter deficiency. SLC6A8-specific criteria applied, as specified by the ClinGen CCDS VCEP (Specifications Version 1.1.0): PP1_Strong, PP4_Strong, PP3, PM2_Supporting. (Classification approved by the ClinGen Cerebral Creatine Deficiency Syndromes Variant Curation Expert Panel on May 11, 2023)