1 + 1 = r4 and much much more.

Expression and mutational analysis has shown that the vertebrate Hox genes are instrumental in patterning of the developing embryo. However, the combined effects of functional redundancy, compensation, and synergy often obscure the precise roles of these genes. By combining gene targeting strategies with the analysis of regulatory sequences from the Hoxa1 and Hoff1 genes, it has been possible to bypass some of these complications and demonstrate their genetic and functional interactions during the development of the hindbrain and branchial arches.