We show that TACI expression is retained on multiple myeloma cells when BCMA expression is lost in both knockout cell lines and in a patient with relapsed multiple myeloma plasmacytoma after treatment with anti-BCMA CAR, further supporting the use of TACI as an alternative CAR T cell target in BCMA-negative malignancies. This evidence concerns the gene TNFRSF17 and plasma cell myeloma.