A cassette flanked by mutant loxP sites was constructed with the entire coding sequence of the target gene oriented in one direction and a second "marker" gene orientated in the opposite direction. This construct was inserted into the first coding exon so that gene expression was simultaneously disrupted and rescued by the inserted cassette. The sequence between the mutant loxP alleles is inverted upon Cre recombinase activity, leading to inactivation of the gene coding sequence and expression of the marker gene.