Hoxa2 expression is driven by Hoxb1 in rhombomere 4 (r4) (Tumpel et al. 2007, Lampe et al. 2008) and Egr2 (Krox20) in r3 and r5 (Maconochie et al. 2001). Hoxb1 together with Pbx and Prep/Meis cofactors bind an element in the intron of Hoxa2 (Tumpel et al. 2007, Lampe et al. 2008, Tumpel et al. 2008). Hoxa2 also autoregulates in r4 (Tumpel et al. 2007, Lampe et al. 2008). (Hoxa2 is the only Hox gene active in r2 (Lampe et al 2004). An unknown activator, possibly a transcription factor of the Sox family, may be involved in expression in r2 (Tumpel et al. 2008)).