Using recombinant proteins, both Pbx1 and E2a-Pbx1 heterodimerize with Hox proteins on bipartite elements, Pbx1 binding a 5' TGAT core and Class I Hox proteins binding adjacent 3' TAAT, TTAT, or TGAT cores.     ///    "In vitro, the PCE fails to bind heterodimers of Class I Hox proteins plus either Pbx1 or E2a-Pbx1. "    ///    " Oncoprotein E2a-Pbx1 contains the N-terminal transactivation domains of E2a and the majority of the homeodomain protein, Pbx1. "