This invention relates to the purification, cloning and characterization of a novel gene, Gab2. In response to extracellular stimuli (e.g., cyokines, growth factors, hormones and antigens), Gab2 binds several signal relay molecules, including the protein-tyrosine phosphatase SHP-2 and phosphatidylinositol-3-OH kinase (PI-3K), which results in the initiation of multiple signaling cascades. Gab2 nucleic acid molecules, peptides, vectors, host cells, probes, antibodies, knockout and transgenic animals are provided. The invention also relates to methods of diagnosis, prevention and treatment of Gab2-mediated conditions such as allergic responses, neoplastic disorders and immune disorders. The invention further relates to diagnostic kits for disorders associated with altered Gab2 expression.