It was demonstrated in mice that homozygous PCP mutations caused craniochisichisis, as shown for Vangl2, Celsr1, and Ptk7 while double heterozygosity for a PCP mutation and non-PCP mutation can cause spina bifida, as shown for Vangl2/Dact1 double heterozygotes. The gene discussed is PTK7; the disease is spina bifida.