5.9 kb of genomic DNA encompassing the CpG island and first five exons of the gene were replaced with a loxP-flanked hygromycin-thymidine kinase selection cassette. Transient expression of cre recombinase in correctly targeted ES cells excised the selection cassette, leaving behind a single loxP site. Gene inactivation was confirmed by immunoblotting of testicular lysates. (J:140402)