In mouse embryos (Ferretti et al. 2000) and embryonal carcinoma cells (Ferretti et al. 2000, Ferretti et al. 2005), Hoxb1 binds with the Pbx1:Pknox1 (Pbx1:Prep1) heterodimer and Meis1 (Huang et al. 2005) at the promoter of the Hoxb1 gene to maintain expression after initial activation of Hoxb1 by retinoic acid signaling. Binding of SOX:OCT heterodimers to the mouse HOXB1 promoter also appears to positively regulate HOXB1 transcription (Di Rocco et al. 2001).