The investigators have demonstrated that levels of the target antigen CD38, and expression levels of the complement inhibitory proteins CD55 and CD59 determine the susceptibility of the MM cells towards daratumumab. In addition, MM cells have lower CD38 expression levels and higher levels of CD55/CD59 at the time of progression. Importantly, all-trans retinoic acid (ATRA) upregulates CD38 levels and downregulates CD55/CD59 levels on MM cells, both in daratumumab naïve cells and in cells that are resistant to daratumumab because of previous exposure to this drug. These alterations in expression explain the strong synergy between ATRA and daratumumab, both in MM cells derived from daratumumab naïve patients and from patients with daratumumab-refractory disease.