Hoxa2 expression in embryos is driven by Egr2 (Krox20) in rhombomeres 3 and 5 (r3, r5) (Maconochie et al. 2001) and by Hoxb1 in r4 (Tumpel et al. 2007, Lampe et al. 2008). Egr2 binds sites in the 5' region of the Hoxa2 gene (Maconochie et al.2001). (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)).