Thick filament-associated protein located in the crossbridge region of vertebrate striated muscle a bands. In vitro it binds MHC, F- actin and native thin filaments, and modifies the activity of actin- activated myosin ATPase. It may modulate muscle contraction or may play a more structural role. This is the function of MYBPC3 (myosin binding protein C3, Ensembl gene identifier ENSG00000134571).