The curly tail (ct) mutant is homozygous for a hypomorphic allele of the transcription factor grainyhead-like-3 (Grhl3) and exhibits spina bifida with 15–20% penetrance (Gustavsson et al., 2007; Ting et al., 2003; van Straaten and Copp, 2001). This evidence concerns the gene GRHL3 and spina bifida.