CD38 expression levels on primary MM cells before start of therapy are significantly higher in responders vs. nonresponders in GEN501 and SIRIUS studies. CD38 expression levels were analyzed by flow cytometry in 102 BM samples obtained from relapsed/refractory MM patients before start of treatment with daratumumab monotherapy at a dose of 16 mg/kg in the GEN501 and SIRIUS studies. HuMax-003 FITC was used as CD38 antibody. Shown are median, 25th-75th percentile (box), and minimum and maximum value (whiskers). CD38 expression levels on MM cells were compared between responders (defined as partial response or better) and nonresponders. P values between the indicated groups were calculated using the Mann-Whitney U test. Every dot represents a patient. N, number.