Notch is one of the neurogenic genes of Drosophila controlling the decision between ectodermal and neural fate for cells in the early embryo. We have used a polymerase chain reaction (PCR)-based strategy to identify cDNA clones representing two mouse homologues to the Drosophila Notch gene. One of the genes, Motch A, is the mouse orthologue to the previously cloned Notch genes in Xenopus, rat, and man. The other gene, Motch B, is more distantly related to the characterized vertebrate Notch genes, but of equal homology to the Drosophila Notch as is Motch A, and therefore represents a distinct branch of a vertebrate Notch gene family. The Motch A and Motch B branches probably arose by gene duplication early in vertebrate evolution. Both Motch A and Motch B retain the same principal structure, encode mRNAs of approximately 10 kilobases, are expressed during mouse embryogenesis, and have largely overlapping expression patterns in adult tissues. We discuss possible consequences of expressing two closely related Notch homologues in the same set of tissues in terms of cell-cell signaling and differentiation control.