All nine expected genotypes occurred at approximately Mendelian frequencies, and, in E10.5 litters, all embryos of Zic2Ku/Ku genotype had open spina bifida, with lack of DLHPs, irrespective of whether they were wild-type, heterozygous or homozygous for the Shh null mutation (Fig. S7). Here, SHH is linked to spina bifida.