Low CRBN expression is common in lenalidomide refractory patients. (A) Nine MM patients, whose pretreatment and post-treatment samples were available, were analyzed by real-time PCR for the expression of CRBN in MM cells collected at both diagnosis (D) and relapse (R) stages. The data for each sample represent the mean values of 4 independent experiments (mean ± SD). (B) An analysis of CRBN expression across different differentiation stages of normal B cells and a variety of B-cell tumors, including marginal zone lymphoma (MZL, n = 86), lymphoplasmacytic lymphoma/Waldenström macroglobulinemia (LPL/WM, n = 21), follicular lymphoma (FL, n = 38), diffuse large B-cell lymphoma (DLBCL, n = 73), MM (n = 238), and HMCLs (n = 60), shows a similar expression level across entities without low expression outliers. Arrows indicate the CRBN expression level in 4 HMCLs that show resistance to lenalidomide (OCIMY5, OPM1, SKMM2, and KMS12 PE). (C) The low CRBN gene expression level found in OCIMY5 and OPM1 was correlated with low protein expression level.