A loxP flanked Hprt selection cassette upstream of exon1 and an additional loxP site downstream of exon 2 are inserted by homologous recombination. Transient Cre expression deleted exons 1 and 2. Although the cre-mediated deletion is predicted to result in the production of an in-frame transcript, the gene product from this allele is predicted to be non functional. Loss of protein was confirmed by immunostaining in early embryos.