Identification of growth factors in neoplasias may be a target for future therapies by blocking either growth factor receptor interaction or the induced pathway. Using gene expression profiling, we identified overexpression of 2 receptors for a proliferation-inducing ligand (APRIL) and B-cell activating factor (BAFF) in malignant plasma cells compared with normal plasma cells. APRIL and BAFF are involved in a variety of tumor and autoimmune diseases, including B-cell malignancies. We confirmed the expression of BAFF and APRIL receptors (B-cell maturation antigen [BCMA], transmembrane activator and calcium modulator and cyclophilin ligand interactor [TACI], and BAFF-R) in a majority of 13 myeloma cell lines and in the purified primary myeloma cells of 11 patients. APRIL and BAFF were potent survival factors for exogenous cytokine-dependent myeloma cell lines and were autocrine growth factors for the RPMI8226 and L363 autonomously growing cell lines. These factors activated nuclear factor (NF)–κB, phosphatidylinositol-3 (PI-3) kinase/AKT, and mitogen-activated protein kinase (MAPK) kinase pathways and induced a strong up-regulation of the Mcl-1 and Bcl-2 antiapoptotic proteins in myeloma cells. BAFF or APRIL was also involved in the survival of primary myeloma cells cultured with their bone-marrow environment, and protected them from dexamethasone (DEX)–induced apoptosis. Finally, the serum levels of BAFF and APRIL were increased about 5-fold in patients with multiple myeloma (MM) as compared with healthy donors. Altogether, these data suggest that APRIL/BAFF inhibitors may be of clinical value in MM. (Blood. 2004;103:3148-3157)