These data form the preclinical rationale for clinical evaluation of ATRA and daratumumab in MM patients. The investigators will treat relapsed/refractory MM patients in two stages. The first stage (part A) consists of treatment with daratumumab monotherapy. In case these patients have progressive disease after cycle 1, less than minimal response after cycle 2, or less than partial response after cycle 3 (unless ongoing response) to single agent daratumumab, or in case these patients progress during daratumumab therapy after previous response, then ATRA will be added to daratumumab (part B).