Patients with high gene expression levels of MCT1 showed significantly reduced PFS (31.9 vs. 48.2 months in MCT1high vs. MCT1low, P=.03) and OS (75.9 months vs. not reached (NR) months in MCT1high vs. MCT1low; P=.001) in case of lenalidomide maintenance...Together, we establish MCT1-expression as a predictive marker for response to lenalidomide-based maintenance treatment.