MYH3 encodes myosin heavy chain 3, an embryonic isoform of skeletal muscle myosin that functions as a molecular motor for muscle contraction through actin filament-based movement and ATP hydrolysis 1. During early muscle development, MYH3 is expressed in fast-twitch skeletal muscle fibers and plays a critical role in sarcomere organization and muscle filament sliding 2. Pathogenic variants in MYH3 cause multiple congenital contracture syndromes inherited in both dominant and recessive patterns 3. These include distal arthrogryposis types 2A and 2B3, Sheldon-Hall syndrome, Freeman-Sheldon syndrome, and contractures, pterygia, and spondylocarpotarsal fusion syndromes (CPSFS) 4 5. Affected individuals present with congenital joint contractures, variable vertebral segmentation anomalies, short stature, and dysmorphic features 3. MYH3-associated disorders represent ultra-rare conditions with non-progressive contractures most severe at birth 4. Pathogenic mechanisms involve disruption of canonical and non-canonical TGF-β signaling pathways, with monoallelic variants decreasing SMAD3 phosphorylation and biallelic variants potentially causing protein truncation 3. Notably, patients typically exhibit normal life expectancy and cognitive abilities, with management focused on early physical therapy, serial casting, and surgical intervention 4.