Dos/Gab family scaffolding adapters (Dos, Gab1, Gab2) bind several signal relay molecules, including the protein-tyrosine phosphatase Shp-2 and phosphatidylinositol-3-OH kinase (PI(3)K); they are also implicated in growth factor, cytokine and antigen receptor signal transduction. Mice lacking Gab1 die during embryogenesis and show defective responses to several stimuli^,. Here we report that Gab2^-/- mice are viable and generally healthy; however, the response (for example, degranulation and cytokine gene expression) of Gab2^-/- mast cells to stimulation of the high affinity immunoglobulin-ε (IgE) receptor FcεRI is defective. Accordingly, allergic reactions such as passive cutaneous and systemic anaphylaxis are markedly impaired in Gab2^-/- mice. Biochemical analyses reveal that signalling pathways dependent on PI(3)K, a critical component of FcεRI signalling, are defective in Gab2^-/- mast cells. Our data identify Gab2 as the principal activator of PI(3)K in response to FcεRI activation, thereby providing genetic evidence that Dos/Gab family scaffolds regulate the PI(3)K pathway in vivo. Gab2 and/or its associated signalling molecules may be new targets for developing drugs to treat allergy.