The molecular lesion in the head slant mouse is a single insertion of a deoxyadenosine residue into exon 1, 28 nucleotides from the start codon. This frameshift mutation is predicted to result in a truncation of the encoded protein after the 34th amino acid. In addition, the transgenic expression of the wild-type gene rescued the defects in the homozygous mutant mouse. (J:106332)