This is the first description of active clinical manifestation of endocardial fibroelastosis (EFE) and remodeling of the endocardium <i>via</i> endothelial-to-mesenchymal transformation (EndMT) in an adolescent with Shone's variant hypoplastic left heart complex (HLHC) and a genetic heterozygous ABL1 variant. This evidence concerns the gene ABL1 and endocardial fibroelastosis.