Tumor-intrinsic resistance mechanisms, including mutational events and immunoselection of mutant clones, were identified in 30 patients with multiple myeloma who experienced relapse after treatment with anti-BCMA CAR T cells and/or anti-BCMA bispecific T cell engagers (TCEs) or anti-GPRC5D TCE therapy. The gene discussed is TNFRSF17; the disease is AL amyloidosis.