CD38 and plasma cell myeloma: In summary, we have demonstrated that a nanobody recognizing an epitope on CD38 distinct from that of daratumumab allows the specific detection of myeloma cells pretreated with daratumumab in vitro, ex vivo, and in vivo. Future studies using radiolabeled nanobody JK36 are warranted to investigate the potential for clinical imaging of daratumumab-pretreated human multiple myeloma patients.