The gene was replaced with a construct composed of a loxP-flanked stop box element within the promoter region and a gene carrying a threonine to alanine substitution at position 187 (T187A). The lox-STOP-lox element was removed by breeding homozygous animals to a line expressing cre recombinase constitutively in the germ line thereby allowing expression of the mutant allele in the next generation.