Gene targeting studies have indicated that the two receptors for PDGF, α and β, direct unique functions during development. Distinct ligand affinities, patterns of gene expression, and/or mechanisms of signal relay may account for functional specificity of the two PDGF receptor isoforms. To distinguish between these factors, we have created two complementary lines of knockin mice in which the intracellular signaling domains of one PDGFR have been removed and replaced by those of the other PDGFR. While both lines demonstrated substantial rescue of normal development, substitution of the PDGFβR signaling domains with those of the PDGFαR resulted in varying degrees of vascular disease. This observation provides a framework for discussing the evolution of receptor tyrosine kinase functional specificity.