MYL1 encodes a non-regulatory myosin light chain essential for proper myofiber formation and muscle contraction. As a structural component of the myosin II complex, MYL1 facilitates muscle filament sliding and sarcomere organization, with particular importance for fast-twitch type II fiber development 1. The gene produces multiple isoforms through alternative promoters and splicing, with expression primarily in skeletal muscle tissues 2. MYL1 variants cause Congenital Myopathy 14, a severe condition characterized by early-onset hypotonia and weakness requiring ventilatory support. Affected individuals show selective type II fiber hypotrophy with aberrant myofibrillogenesis and autophagic impairment, revealing MYL1's broader role in muscle fiber organization across all fiber types 1. Additionally, an MYL1 intron polymorphism (rs1472955) associates with endurance trainability in cardiac muscle, with AA carriers showing greater cardiac adaptation to endurance training than GG carriers 3. Beyond congenital myopathy, MYL1 dysregulation appears in multiple cancer types. In rhabdomyosarcoma and head and neck squamous cell carcinoma, upregulated MYL1 correlates with poor prognosis and increased tumor metastasis, potentially through EGF/EGFR pathway activation [PMID:33499774; 46]. MYL1 peptides are also elevated in amyotrophic lateral sclerosis cerebrospinal fluid, serving as a potential biomarker for disease discrimination 5.