All the cells in all animals have a complete set of genes with the instructions for building an organism. Working with mutations in fruit flies, scientists have found a group of regulatory genes called Hox genes that signal other genes in sequence to build the body of an animal. All animals except sponges have Hox genes.
During development, a special set of regulatory genes, called Hox genes, directs the other genes to build the organism. In this way, cells differentiate to become body traits. Understanding how these regulatory genes work helps us understand how new body plans evolved.