The gene locus was inactivated by inserting the enhanced green fluorescence protein (GFP) coding sequence in-frame into the ATG-start site of Eki2, replacing part of exon 1 and exon 2. A loxP-flanked neo selection cassette was included downstream of the GFP cassette and was subsequently removed by crossing mice with CMV-cre transgenic mice. Gene inactivation was confirmed by quantitative PCR of testis RNA. GFP expression was observed in the testis of E13.5 embryos and adult males. (J:142456)