MYMK encodes myomaker, a plasma membrane protein essential for myoblast fusion during skeletal muscle development and regeneration 1. The protein mediates the fusion of myoblasts into multinucleated muscle fibers by facilitating membrane lipid mixing (hemifusion) upstream of MYMX 1. MYMK is expressed in specific satellite cell subpopulations and functions during muscle repair by enabling fusion of satellite cells with injured myofibers 2. Loss-of-function mutations in MYMK cause Carey-Fineman-Ziter syndrome (CFZS), a rare congenital myopathy characterized by facial weakness, hypotonia, and motor delays 1. Zebrafish knockout studies demonstrate that mymk deficiency results in defective myoblast fusion, predominantly mononucleated muscle fibers, reduced muscle growth, and increased adipocyte infiltration 3. Paradoxically, CFZS patients exhibit muscle fiber hypertrophy with increased myonuclei but disproportionally larger myonuclear domains and reduced contractile force generation 4. MYMK expression is regulated by developmental signals and is part of gene signatures associated with muscle regeneration across various myopathies 5. The protein's critical role in muscle fusion makes it essential for normal muscle development, maintenance, and therapeutic regeneration.