Expression and activation status of the cyclin D1 target proteins CDK4 and RB in MCL cells. Expression of CDK4, CDK6, and RB (both total and phosphorylated at the depicted serines) by MCL cell lines (A) and primary MCL samples (B). (C) Persistent association of cyclin D1 isoforms with CDK4 in MCL cells. Cell lysates from 2 MCL cell lines (Jeko-1 and SP53) and one MCL patient (patient16) immunoprecipitated with the anti-CDK4 antibody and probed in Western blot with antibodies against cyclin D1a (top row), cyclin D1b (middle row), and CDK4 (bottom row). Detected protein bands are depicted in the rows designated IP pellet. Expression of the proteins in the cell lysates not subjected to the CDK4 antibody precipitation served as the control (rows designated as input). (D) Expression of the p16 protein by the depicted MCL cell lines and primary tumor samples. Cell lysate from the p16 protein-expressing SUDHL-1 cell line used at the matching protein concentration (first lane) and after 1:20 dilution (second lane) served as control. Results are representative of at least 2 independent experiments.