A targeting vector was designed to insert a loxP site upstream of the start codon, and a second loxP site just downstream of exon 3 of the v-myc myelocytomatosis viral oncogene homolog 1 (Mycl1) targeted gene. A FRT-flanked neomycin (neo) resistance cassette was located upstream of the second loxP site. Flp mediated recombination removed the neo cassette.