FOS regulates MAF transcription. (A) Chromatin immunoprecipitation of FOS demonstrates binding to MAF promoter in MM cells that is decreased with MEK inhibition (U0126, 10μM). (B) MEK inhibition (U0126, 10μM) decreased FOS binding to MAF locus by chromatin immunoprecipitation in MAF-expressing cell lines. (C) Knockdown of FOS is toxic to MAF-overexpressing myeloma cell lines. The indicated myeloma cell lines were transduced with a retrovirus expressing FOS shRNA and coexpressing GFP (“Retrovirally mediated transduction”). Live GFP⁺ cells were enumerated by fluorescence-activated cell sorter and normalized to the value at day 2 after retroviral infection. Reexpression of MAF rescues myeloma cells transduced with an shRNA targeting FOS. (D) Quantitative RT-PCR measurement of FOS mRNA levels after induction of the FOS shRNA in H929 cells. (E) Western blot of FOS protein levels after introduction of the FOS shRNA in H929 cells.