Arginine codon 172 (CGC) was changed to histidine (CAC) (c.515G>A, p.R172H, equivalent to p.R175H in humans) and a loxP site flanked neomycin resistance gene cassette was inserted into intron 4. The neo cassette was excised via Cre-mediated recombination. Western blot analysis of homozygous MEFs detected mutant protein. (J:95318)