The protooncogene MYB is predominantly expressed in immature hemopoietic cells where it has an essential role in hemopoietic cell proliferation and differentiation. Oncogenically activated forms of MYB is generally N- and/or C-terminal truncations of the normal MYB protein. Removal of the C terminus of MYB disrupts or deletes a region termed the negative regulatory domain (NRD), resulting in an increase in DNA binding, transactivation, and transformation by MYB. One feature of the NRD is a leucine zipper-like motif [PMID: 8302594]. Murine Myb-binding protein-1a (MYBBP1A), originally called P160, was identified by its ability to interact specifically with Myb via this leucine zipper-like motif. MYBBP1A modulates MYB activity upon binding to the MYB NRD [PMID: 10644447, 9447996].