SDC1 and plasma cell myeloma: Several promising non-BCMA CAR-T targets in MM have emerged, including CD138 (syndecan-1), which is abundantly expressed on myeloma cells [44]; GPRC5D (G-protein-coupled receptor, class C, group 5, member D), a receptor implicated in cell proliferation and found on myeloma cells [45]; and FcRH5 (Fc receptor homolog 5), a receptor linked to the immune response present on myeloma cells [46].