Hoxb1 in a trimeric complex with Pbx1 and Meis1 or Pknox1 (Prep1) binds an enhancer located 5' to the Hoxb2 gene and activates expression of Hoxb2 in rhombomere 4 (r4) of embryos (Maconochie et al. 1997, Jacobs et al. 1999, Ferretti et al. 2000, Ferrai et al. 2009). Egr2 (Krox20) activates Hoxb2 in r3 and r5 and expression is also observed in r6 and r7.