Importantly, 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.