This phenotypic mutation was identified in an ENU mutagenesis screen at the MRC Mammalian Genetics Unit, Harwell, UK. A point mutation in Zfhx3 was identified in exon 9, resulting in a G to T transversion at position 6620 and a substitution of a phenylalanine for a valine at residue 1963 (V1963F) in a highly conserved region just upstream of the 17th zinc-finger motif. This appears to be a gain of function allele.