A targeting vector was designed to insert loxP sites flanking exons 2-4 of the Dickkopf-Like 1 (Dkkl1) gene, and a frt-flanked neomycin resistance (neo) cassette upstream of the 3' loxP site. Cre mediated recombination removed exons 2 through 4 and the frt-flanked neo cassette. The absence of protein expression was confirmed by immunohistochemistry on testis samples.