Many growth factors function by binding receptors with intrinsic tyrosine kinase activity. Signaling by such receptors involves a series of intermediates characterized by SH2 domains that bind tyrosine phosphorylated receptors by a direct interaction between the SH2 domain and specific receptor sequences. For instance, the GRB family of proteins lack a defined catalytic activity and are entirely composed of SH2 and SH3 domains. Members include GRB2, GRB7, GRB10 (also referred to as GRB-IR), GRB14 and Grap (for GRB2-related adapter protein). While GRB10 and GRB14 are most closely related to GRB7, Grap shares the highest degree of homology with GRB2 exhibiting 59% sequence identity with GRB2. The Grap SH2 domain is capable of binding to the activated stem cell factor receptor, c-Kit and the erythropoietin receptor (EpoR). Grap also associates with the Ras guanine nucleotide exchange factor Sos 1 via its amino terminal SH3 domain.