The NM_005629.4:c.92del (p.Pro31ArgfsTer?) variant in SLC6A8 is a frameshift variant predicted to cause a premature stop codon in biologically-relevant-exon 2/13 leading to nonsense mediated decay in a gene in which loss-of-function is an established disease mechanism (PVS1). An adult female, heterozygous for the variant, has been reported with IQ in the low normal range, normal urine creatine level and 85% normal creatine level on brain MRS. Her son is reported to be affected but individual clinical and biochemical data is unavailable PMID: 20528887, 23644449). There is insufficient data to apply PP4. The variant is absent in gnomAD v2.1.1. (PM2_Supporting). The classification of this variant has been upgraded from Variant of Uncertain Significance to Likely Pathogenic based on the recommendations of the ClinGen Sequence Variant Interpretation Working Group, that a variant meeting PVS1 and PM2_Supporting is classified as Likely Pathogenic (https://clinicalgenome.org/site/assets/files/5182/pm2_-_svi_recommendation_-_approved_sept2020.pdf )In summary, this variant meets the criteria to be classified as likely pathogenic for X-linked creatine transporter deficiency. SLC6A8-specific ACMG/AMP criteria met, as specified by the ClinGen Cerebral Creatine Deficiency Syndromes VCEP: PVS1, PM2_Supporting. (Classification approved by the ClinGen Cerebral Creatine Deficiency Syndromes Variant Curation Expert Panel on March 28, 2024).