The Grb2-associated binder (Gab) family adapter proteins are scaffolding adapter molecules that display sequence similarity with Drosophila DOS (daughter of sevenless), which is a substrate for the protein tyrosine phosphatase Corkscrew. Gab proteins contain a pleckstrin homology (PH) domain and binding sites for SH2 and SH3 domains. A number of studies in multiple systems have implicated Gab in signaling via many different types of receptors, such as growth factor, cytokine, and antigen receptors, and via oncoproteins. Recent studies of Gab1 and Gab2 knockout mice have clearly indicated an important role for Gabs in vivo. Gab1-deficient mice die as embryos with multiple defects in placental, heart, skin, and muscle development. Gab2-deficient mice are viable, but have a defect in the mast cell lineages and in allergic reactions. Given the apparently central role played by Gab signaling via many receptors, delineating the precise mechanism(s) of Gab-mediated signaling is critical to understanding how cytokines, growth factors, and oncoproteins mediate a variety of biological activities: cell growth, differentiation, survival and malignant transformation.