The NM_005629.4:c.1540C>T (p.Arg514Ter) variant in SLC6A8 is a nonsense variant that is predicted to cause a premature stop codon in biologically-relevant-exon 11/13, leading to nonsense mediated decay in a gene in which loss-of-function is an established disease mechanism (PVS1). The variant is absent in gnomAD v2.1.1. (PM2_Supporting). A male patient has been reported with almost complete absence of the creatine signal on brain MRS, elevated creatine in urine, and virtually undetectable creatine transport in fibroblasts (PMID: 11326334) (PP4_Strong). The mother, maternal aunt, and maternal grandmother are all heterozygous for the variant and all have evidence of creatine transporter deficiency. The paternal uncle has severe intellectual disability but was unavailable for study (PMID: 11326334) (PP1_Strong). The variant is absent in gnomAD v2.1.1. (PM2_Supporting). In summary, this variant meets the criteria to be classified as pathogenic for X-linked creatine transporter deficiency. SLC6A8-specific ACMG/AMP criteria applied, as specified by the ClinGen Cerebral creatine Deficiencies Variant Curation Expert Panel CCDS VCEP) (Specifications Version 1.1.0): PVS1, PP1_Strong, PP4_Strong, PM2_Supporting. (Classification approved by the ClinGen CCDS VCEP on Oct. 13, 2023)