Exon 4 was replaced with the following sequences: a loxP site, a partial cDNA (from E330037C1Rik) containing exons 4-19, an FRT site flanked neomycin resistance gene cassette, a second loxP site, exon 4 splice acceptor site, the EGFP reporter gene, and a STOP cassette. The neo cassette was removed through subsequent flp-mediated recombination and the cDNA insert through cre-mediated recombination. This leaves a knockout reporter allele.