MYBPHL encodes myosin binding protein H-like (MyBP-HL), a myofilament-associated protein that plays a critical role in cardiac conduction and atrial function 1. The protein is preferentially expressed in atrial tissue, with high atrial expression and low ventricular expression, though it is also found in discrete puncta throughout the ventricular conduction system, particularly near the atrioventricular node and Purkinje fibers 2. MyBP-HL functions by incorporating into myofilaments and regulating calcium handling in atrial cardiomyocytes 2. Loss-of-function mutations cause dilated cardiomyopathy, atrial enlargement, and cardiac conduction defects including atrioventricular block and atrial arrhythmias 12. Mechanistically, MYBPHL deficiency leads to ryanodine receptor disorganization, increased calcium release heterogeneity, and shortened atrial refractory periods 2. Clinically, MYBPHL serves as a specific biomarker for atrial damage, with rapid release into circulation following atrial ablation procedures 3. The protein exists in two isoforms, with isoform 2 being almost exclusively atrial-specific 3. Beyond cardiac function, MYBPHL variants have been associated with multiple myeloma pathogenesis through enhancer hypomethylation mechanisms 4.