HLCS (Holocarboxylase Synthetase): HLCS is an enzyme essential for the proper functioning of carboxylase enzymes, which play key roles in protein, carbohydrate, and lipid metabolism. Mutations in HLCS can cause holocarboxylase synthetase deficiency, a rare metabolic disorder characterized by multiple carboxylase deficiencies, leading to issues with skin, hair, immune function, and developmental delays.