Negatively regulates B-cell proliferation following stimulation through the B-cell receptor. May play an important role in maintenance of marginal zone (MZ) B-cells. Interacts with GRB2. Expressed primarily in B220+ splenocytes and total bone marrow cells. Expressed at lower levels in mast cells and dendritic cells. Not detected in T-cells and macrophages (at protein level). Mice are normal in appearance, size and fertility. In aged mice, the number of MZ B-cells is increased, and serum concentrations of IgM, IgG2b, and IgG3 are elevated. Belongs to the GAPT family.