The NM_001100.4:c.124C>T variant in ACTA1 is a missense variant predicted to cause substitution of histidine by tyrosine at amino acid 42 (legacy nomenclature: p.His40Tyr). This variant is absent from gnomAD v4.1 (PM2_Supporting). The computational predictor REVEL gives a score of 0.72, which is above the threshold of 0.7, evidence that correlates with impact to ACTA1 function (PP3). ACTA1, in which the variant was identified, is defined by the ClinGen Congenital Myopathies VCEP as a gene that has a low rate of benign missense variation and where pathogenic missense variants are a common mechanism of disease (PP2). In vitro studies demonstrated changes in actin localization and motility (PMID:15226407, 17227580, 27112274). In vivo studies in a mouse model demonstrated impaired function of ACTA1 and limited intrinsic force-generating capacity and maximal tetanic force (PMID:23613869, 23656990)(PS3). This variant has been reported in 4 probands/families with intranuclear rod and/or nemaline myopathies (PS4_Moderate; PMID:10508519, 12921789, 19562689). This variant has occured de novo with confirmed parental relationships in 1 individual and was reported to occur de novo with unconfirmed parental relationships in 1 individual with severe atypical nemaline myopathy with intranuclear nemaline bodies (PS2, PP4_Moderate; PMID:10508519, 19562689). In summary, this variant meets the criteria to be classified as pathogenic for autosomal dominant alpha-actinopathy based on the ACMG/AMP criteria applied, as specified by the ClinGen Congenital Myopathies VCEP: PS2, PS3, PS4_Moderate, PP4_Moderate, PM2_Supporting, PP2, PP3 (Congenital Myopathies VCEP specifications version 2; 08/27/2024.