HLCS (holocarboxylase synthetase) is a biotin–protein ligase that catalyzes the covalent attachment of biotin to four biotin-dependent carboxylases: acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and methylcrotonyl-CoA carboxylase 1. These carboxylases are essential enzymes involved in cellular metabolism of fatty acids and amino acids, and gluconeogenesis 2. HLCS also biotinylates histones, linking biotin metabolism to gene regulation 1. Biotin, a water-soluble B-vitamin cofactor, is not synthesized endogenously and must be obtained from dietary sources or intestinal bacterial production 2. HLCS deficiency is an autosomal recessive disorder characterized by defective biotin incorporation into carboxylases, leading to impaired enzyme function 1. Clinical manifestations include neurological and dermatological symptoms, with disease severity correlating with residual HLCS enzyme activity 1. Patients with mutations producing minimal residual activity present with neonatal onset, while those with higher residual activity develop later-onset symptoms 1. High-dose biotin therapy shows clinical efficacy in HLCS deficiency patients 2, with responsiveness directly related to preserved enzyme function 1. Over 30 HLCS mutations have been identified, distributed across the coding region, including population-specific founder mutations in Scandinavian and Japanese populations 1.