TheHox genes encode a class of transcription factors that are thought to be crucial for specification of body plans. This means that they are of interest to both developmental and evolutionary biologists, and differences in the expression of these genes could explain the evolution of animal phyla. In general, invertebrates have only one Hox gene cluster, whereas vertebrates usually have four, leading to the prediction that the number of clusters increases with increasing complexity. But a new study casts doubt on this theory, showing that the zebrafish may actually have six Hoxgene clusters.
