The second exon of Mterf1a was flanked by a loxP site and an FRT-flanked neomycin resistance cassette with a loxP site. The second exon of Mterf1b was replaced with a hygromycin resistance cassette. ES cells were selected for both mutations on the same chromosome. Flp-mediated recombination removed the neomycin resistance cassette. Cre-mediated recombination removed the second exon of Mterf1a. Western blot analysis confirmed the absence of protein expression in the liver.