MAST4 and epilepsy: This case report broadens the known phenotypic spectrum by characterizing a novel p.Arg1381Leu variant and its unique association with myelination deficits in the absence of epilepsy—a departure from previously reported cases.The observed phenotypic heterogeneity likely reflects a complex interplay between the primary variant and modifier gene effects, particularly through epigenetic regulation of MAST4 expression during neurodevelopment.