Protein interaction partners of CCND1: Different gene expression in MCL subgroups. The colors red, blue and grey mean "over expressed", "down regulated" (in "b") and "not available in the data set". FOS encodes for a leucine zipper protein and plays a role in regulation of cell proliferation, differentiation, transformation and tumorigenesis . The JUN protein interacts directly with specific target DNA sequences to regulate gene expression  and is involved in tumorigenesis by cooperating with oncogenic alleles of Ras, an activator of the mitogen activated protein kinases . MYC and MYBL2 play a role in cell cycle progression and act as transcription factors. MYC is also associated with apoptosis, cellular transformation, cell growth, proliferation, differentiation, and a variety of hematopoietic tumors, leukemias and lymphomas [61, 62, 63], and was part of the original proliferation signature . MYBL2 has been shown to play a role in the G1/S transition  and proliferation  and is known to be regulated by CCND1 [66, 67]. CDK4 and CDK6 are important regulators of cell cycle transition from G1 to S, phosphorylate, and thus regulate the activity of tumor suppressor protein Rb .