PDGFRA and Spina bifida occulta: Severe spina bifida occulta has previously been observed in Pdgfra null and signaling-deficient mouse mutants (Hamilton et al., 2003; Klinghoffer et al., 2002; Soriano, 1997) and in Pdgfa; Pdgfc double knockout mutants (Ding et al., 2004).