Crystal structure of integrin αVβ3 showing the dimer and individual subunits . The domains that make up each integrin subunit are shown. Secondary structure elements are shown as red α-helices or cyan β-strands/ribbons. Blue circles represent the six cation-binding sites. The plexin–semaphorin–integrin domain and two of the four epidermal growth factor (EGF) repeats in the β-subunit are not visible in the structure.