The coding region of exon 3, which is the last coding exon, was replaced with a neomycin selection cassette inserted by homologous recombination. RT-PCR analysis of homozygous mutant embryos indicated an absence of normal transcript, but identified a mutant transcript containing exons 1 and 2. The mutant transcript lacks exon 3 sequence and therefore does not encode the putative HSPG and FGFR binding sites.