- Local alterations of Krox-20 and Hox gene expression in the hindbrain suggest lack of rhombomeres 4 and 5 in homozygote null Hoxa-1 (Hox-1.6) mutant embryos. Dollé, P., Lufkin, T., Krumlauf, R., Mark, M., Duboule, D., Chambon, P. Proc. Natl. Acad. Sci. U.S.A. (1993)