Pax genes have been cloned on the basis of their homology to the Drosophila segmentation gene paired. They share a common domain, the paired domain, that is sufficient to mediate sequence-specific DNA binding. Thus far, nine members have been characterized, which exhibit highly restricted temporal and spatial expression patterns. The analysis of mouse mutants has revealed their crucial role in the formation of a variety of tissues. In particular, they are involved in the regulation of early steps in organ development. They act to define the regional specification of distinct germ layers.