Exons 2-4 were replaced with a GFP-neo cassette via homologous recombination. The gene targeting event deletes the first 440 amino acids of the protein, which encodes the entire signal sequence and most of the extracellular Sema-like domain. Absence of gene expression in homozygous mutants was confirmed by RT-PCR analysis of RNA from E16.5 embryos and by Western analysis of brain lysates from postnatal day 1 homozygous mutant neonates. (J:72426)