To identify this ligand, we generated Pdgfc−/− mice that were also heterozygous for Pdgfa, Pdgfb or Pdgfd. The only genotype that resulted in a spina bifida occulta with hemorrhage was Pdgfc−/−; Pdgfa+/−, confirming recently published observations (Andrae et al., 2013). The gene discussed is PDGFA; the disease is hemorrhage.