The DNA-binding domain of c-Myb binds the co-repressor protein SIN3A (Nomura et al. 2004). The tumor repressor p53 also binds MYB directly (Tanikawa et al. 2004), promoting formation of a trimeric SIN3A:c-Myb:p53 complex. This does not affect the ability of c-Myb to bind to DNA, but may represent the mechanism that allows p53 to to regulate specific Myb target genes. c-Myb (gene symbol MYB) is highly conserved in all vertebrates and some invertebrate species (Lipsick 1996). It plays an important role in the control of proliferation and differentiation of hematopoietic progenitor cells (Duprey & Boettiger 1985); Down-regulation of c-Myb is believed to be critical for the commitment of cells to terminal differentiation (Oh & Reddy, 1999). c-Myb interacts with many other transcription factors including CBP, several CCAAT binding protein (c/EBP) family members, and Ets family proteins such as Ets-2 (Oh & Reddy, 1999). Loss of c-Myb function results in embryonic lethality due to failure of fetal hepatic hematopoiesis (Mucenski et al. 1991).