An 11.8-kb segment of the gene extending from 27 nucleotides in exon 2 through exon 3 was replaced with a beta-gal reporter gene and a loxP-flanked neomycin selection cassette. Quantitative RT-PCR on 9.5 embryos demonstrated no gene expression in homozygotes. Beta-gal expression was observed in the developing cardiovascular system. (J:125903)