During embryogenesis the Mafb:Jun (Kreisler:Jun) heterodimer binds the rhombomere 5 (r5) enhancer located 5' to the P1 promoter of the Hoxb3 gene and activates expression in r5 (Manzanares et al. 1997, Manzanares et al. 1999, Manzanares et al. 2002, Yau et al. 2002, Mechta-Grigoriou et al. 2003). Egr2 (Krox20) also binds the r5 enhancer and is required to activate the Hoxb3 gene in r5 (Manzanares et al. 2002). After initial activation Hoxb3 is hypothesized to maintain its own expression (Yau et al. 2002). Hoxb3 is expressed most strongly in r5 and more weakly in caudal rhombomeres r6 and r7.