The p.Thr158Ala variant in MECP2 (NM_004992.3) has been reported as a de novo occurrence (biological parentage unconfirmed) in an individual with preserved speech variant Rett syndrome (PMID 11269512) (PM6). The p.Thr158Ala variant has been observed in 2 other individuals with Rett syndrome (PMID 18842453, 15057977) (PS4_moderate, PP4). Multiple pathogenic missense variants have been previously identified within this codon which indicates that this residue is critical to the function of the protein (PMID 18337588, 23270700, 23421866, 10508514, internal database - Invitae) (PM5_strong). The p.Thr158Ala variant in MECP2 is absent from gnomAD (PM2_supporting). Heterochromatin binding and in vitro transcription repression assays have shown that this variant impacts protein function (PMID 12843318) (PS3_supporting). Computational prediction analysis tools suggests a deleterious impact; however, this information does not predict clinical significance on its own (PP3). In summary, the p.Thr158Ala variant in MECP2 is classified as Pathogenic for Rett syndrome based on the ACMG/AMP criteria (PM5_strong, PM6, PS4_moderate, PS3_supporting, PM2_supporting, PP3, PP4).