Prior clinical experience with CD38-targeting antibodies, including daratumumab, establish this molecule as an important therapeutic target in multiple myeloma. CD38 is a type II transmembrane glycoprotein with both ectoenzymatic and signal transductionfunctions. CD38 dimerization is required for its catalytic activity (Desphande et al, 2017).Dimeric CD38 possesses both ADP ribosyl cyclase and cADPR hydrolyase activities; the former generates cyclic ADP ribose (cADPR) from NAD+ and the latter generates ADPR. CD38-mediated cADPR generation enables calcium release from intracellular stores, leading to a variety of cell type-specific physiological outcomes (Schuber and und, 2004; Lee and Aarhus, 1991). A distinct mode of CD38-dependent signal transduction involved in the regulation of proliferation and survival requires interactions with other cell surface molecules (Deshpande et al, 2017; Deaglio et al, 2007; Konopleva et al, 1998). In B- lymphocytes for example, CD38-dependent signal transduction activity utilizes interactions with CD19 and CD31/PECAM-1, the latter of which is the sole identified ligand for CD38 (Deaglio et al, 2007). Importantly, AMG 424 binding to CD38 on target cells results in redirection of T cell mediated cytotoxicity but should not modulate CD38 enzymatic activity or CD38-dependent signal transduction, based on expected serum concentrations in our proposed dosing scheme and significanty reduced binding affinity compared to a monoclonal antibody that does inhibit CD38 activity (Deckert et al., 2014).For this first-in-human study, relapsed/refractory multiple myeloma has been chosen as the indication for clinical investigation of AMG 424 because of the nearly uniform high expression of CD38 on plasma cells observed at the protein level, using flow cytometry (Lin et al, 2004). Although low level CD38 expression is widespread in different tissue types, including hematopoietic cells, plasma cells express CD38 protein at signficantly higher levels than other cell types (Malavasi et al, 2008 and Section 3 of AMG 424 IB).