CDK4 and monoclonal gammopathy: We confirmed increased expression of ERK1/2 (in MM P < 0.05 or 0.04; in plasma cell leukemia (PCL), P < 0.3E-0.07) and CDK4/6 (P < 3.7E-005 or 0.0006; P < 0.02 or 0.05) in clonal PCs with progression from monoclonal gammopathy of undetermined significance (MGUS) to MM and PCL compared to healthy donor PC (NPC) (Fig. 1A, Fig. S1A).