A translational stop codon was introduced at codon 73 in exon 3 in the targeting vector. The vector also contains both a tACE-Cre and neoR module. Recombination in ES cells introduced the stop codon into the Ncf4 sequence. When chimeras were bred, tACE promoter activity in testes promoted Cre/loxP-mediated deletion of the cassette, leaving 59 bp of foreign DNA. No Ncf4 protein can be detected by antibodies in homozygous mice.