The extracellular region of a human integrin. (a) The crystal structure represents a net form of integrin αVβ3 with no bound RGD peptide (Protein Data Bank (PDB) code 1JV2) [3,4]. See PBD code 1L5G for the RGD-bound form. (b) The I (inserted or interactive) domain is present in seven human α subunits between β-propeller repeats 2 and 3, and is involved in ligand binding. An I-like domain is present in all human integrin β subunits along with four EGF-like repeats. Both the I and I-like domains have a Rossmann fold.