4-1BBL crosslinks with the 4-1BB molecule on activated T cells, potentiating selective tumor cell killing.23 To also test these findings for AML cells expressing low levels of the CD33 antigen, the bsAb-releasing hMSCs were further modified to co-express the 4-1BBL cell surface molecule (Figure 3a). The gene discussed is CD33; the disease is acute myeloid leukemia.