Actin capping protein (CP) was named for its ability to bind the barbed ends of actin filaments. CP inhibits the addition and loss of actin subunits at the barbed end and is important for the dynamics of actin filament assembly, and therefore important for the control of cell shape and movement. CP was called ?-actinin when first characterized and purified from muscle (Maruyama 1966). Actin polymerization is controlled by a large cellular excess of capping proteins which bind to the barbed end of actin filaments preventing elongation. Narita et al. (2006) have proposed a two-step binding mechanism by which the fluctuating actin filament end is capped by CP.