A central issue in signal transduction is the physiological contribution of different growth factor-initiated signaling pathways. We have generated knockin mice harboring mutations in the PDGFα receptor (PDGFαR) that selectively eliminate its capacity to activate PI3 kinase (α^PI3K) or Src family kinases (α^Src). The α^PI3K mutation leads to neonatal lethality due to impaired signaling in many cell types, but the α^Src mutation only affects oligodendrocyte development. A third knockin line containing mutations that eliminate multiple docking sites does not increase the severity of the α^PI3K mutation. However, embryos with mutations in the PI3K binding sites of both PDGFRs (α and β) recapitulate the PDGFαR null phenotype. Our results indicate that PI3K has a predominant role in PDGFαR signaling in vivo and that RTK-activated signaling pathways execute both specific and overlapping functions during mammalian development.