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 (By similarity). Interacts with GRB2. Highly expressed in spleen and PBL, detected at lower levels in thymus, and undetectable in all other tissues tested. Also expressed in various B-cell lines, monocytic cell line THP-1 and NK-like cell line YT, but not in T-cell line Jurkat or HeLa cells. Belongs to the GAPT family.