MYL3 encodes the regulatory light chain of myosin, a structural component of the sarcomere that regulates striated muscle contraction and cardiac force generation 1. As a non-calcium-binding regulatory light chain, MYL3 functions within the myosin II complex to modulate cardiac muscle contractility and ventricular morphogenesis. MYL3 is definitively associated with familial hypertrophic cardiomyopathy (HCM), classified among eight genes with definitive evidence of disease causation 1. Pathogenic MYL3 variants account for a small but consistent proportion of HCM cases identified through genetic screening 2, with penetrance of approximately 32% in variant carriers—lower than major HCM genes like MYH7 and MYBPC3 3. Pediatric cardiomyopathy patients harbor a higher burden of MYL3 variants compared to adults, suggesting age-dependent disease manifestation 4. Beyond cardiac function, MYL3 protects chondrocytes from senescence by inhibiting clathrin-mediated endocytosis and Notch signaling activation, with potential implications for osteoarthritis 5. Clinically, MYL3 genetic testing is recommended as part of systematic HCM screening strategies, following primary analysis of MYBPC3 and MYH7 2. The variable penetrance of MYL3 variants necessitates careful genetic counseling regarding disease risk in asymptomatic carriers.