IRES-lacZ reporter cassette as wells as a loxP site were inserted 171 bp upstream miR-204 stem loop and a loxP site plus a neomycin selection cassette were placed 365 bp downstream miRNA stem loops by homologous recombination in ES cells. The chimeric mice where crossed to flippase-expressing transgenic mice to remove Fret or F3 (a mutant of Frt) flanked lacZ and Neo cassettes, leaving two loxP sites immidiately flanking the miRNA stem-loops. (J:276908)