Complete nucleotide sequences are now available for different species of the animal kingdom: Caenorhabditis elegans - a nematode, Drosophila - an insect, and humans - a mammal. Such information makes it possible to compare the set of nuclear receptors found in these organisms, and to discuss the possible reasons for the differences observed. The human genome sequencing identified few new receptors, which implies that most nuclear receptors have now been found. However, information about polymorphisms and regulating sequences, obtained through genomic sequencing, will be important for understanding receptor function and disease mechanisms. The surprisingly large number of nuclear receptors in C. elegans might have implications for the development of pharmaceuticals and the understanding of the function of these animals. By contrast, Drosophila has few nuclear receptors; however, examination of the unique nuclear receptors provides information about the function of these receptors.