HACD1 (3-hydroxyacyl-CoA dehydratase 1) is a muscle-enriched enzyme catalyzing the third step of very long-chain fatty acid (VLCFA) elongation 1. In skeletal muscle, HACD1 regulates membrane lipid composition by modulating fatty acid chain length and saturation, particularly reducing lysophosphatidylcholine levels while increasing C18+ monounsaturated phospholipids 2. These lipid modifications decrease plasma membrane rigidity and promote myoblast fusion during muscle development and regeneration 2. HACD1 functions with partial redundancy alongside HACD2 across saturated and polyunsaturated fatty acid elongation pathways 3. Biallelic loss-of-function HACD1 variants cause congenital myopathy 11, characterized by neonatal-onset generalized muscle weakness, hypotonia, and fiber size disproportion 451. Pathogenic mutations include nonsense variants and splice-site alterations that reduce VLCFA synthesis 4. HACD1 deficiency produces progressive structural defects in muscle membrane systems, including T-tubule dilatation, sarcolemmal abnormalities, and mitochondrial mislocalization, impairing excitation-contraction coupling 6. The clinical presentation typically shows temporally progressive improvement characteristic of HACD1-related myopathy 4. Genetic diagnosis provides important prognostic value for this congenital condition 4.