In vertebrates the entire Hox cluster has been duplicated multiple times. Mice and other mammals have four Hox clusters. All four are similar, but each is different. Similar genes in different clusters are called paralogs.
Most paralogs have partially overlapping functions, so figuring out how Hox genes function in vertebrates a challenge: the effects of changing a single gene are often hidden by functioning genes in the same paralogous group. But changing the function of multiple genes in the group can have dramatic effects.