Pdgfc−/− mice develop a mild form of spina bifida (Ding et al., 2004), but homozygous Pdgfc knockout alone was not sufficient to induce the severe form of spina bifida occulta associated with hemorrhage that we observed in Pdgfc−/−; PdgfraGFP/+ mice (Fig. 1B). The gene discussed is PDGFC; the disease is Spina bifida occulta.