BAC clone CH29-40010 containing the mouse Mir22 gene was manipulated through Red/ET recombination technology. A loxP was inserted in the 5 flanking region of exon 1 and a second loxP site was introduced in the 3 flanking region of exon 1 with a PGK-FRT-Neo-FRT-loxP cassette. Targeted ES cells were re-electroporated with a flp-recombinase expressing plasmid to remove the neomycin cassette leaving exon 1 floxed. Exon 1 was deleted via cre-mediated recombination in the germline. RT-qPCR confirmed undetectable levels of mature miR-22-3p in the liver, epididymal white adipose tissue and gastrocnemius tissue. (J:331713)