The Wnt genes belong to a family of protooncogenes with at least 13 known members that are expressed in species ranging from Drosophila to man. The name Wnt denotes the relationship of this family to the Drosophila segment polarity gene "wingless" and to its vertebrate ortholog, Int-1, a mouse proto-oncogene. Transcription of Wnt family genes appears to be developmentally regulated in a precise temporal and spatial manner. The Wnt genes encode cysteine-rich putative glycoproteins which have