How many receptors does it take?

Three recent reports identify two genes, thick veins (tkv) and saxophone (sax), which encode serine/threonine transmembrane proteins that act as receptors for mediating different aspects of the Drosophila TGF-beta-related signal, dpp. tkv is required for patterning the entire embryonic dorsal region, while sax is required for patterning only amnioserosa, the dorsalmost cell fates. dpp signaling in other developmental processes again requires both tkv and sax, but to differing degrees. tkv and sax encode type I receptors, which appear to directly bind ligand, unlike what has been observed for other type I receptors.