The gene trap construct inserted into intron 1 and consisted of a splice acceptor sequence from the mouse En2 gene, followed by a beta-galactosidase/neomycin (beta-geo) reporter-selection cassette and SV40 polyadenylation signal. The insertion of the gene trap vector inserted 123 bp from the 5-end of the 1066 bp intron 1. The insertional mutation interrupts transcription of the gene in cells and tissues isolated from homozygous mutant mice. Expression of the gene was reduced in cells and tissues heterozygous for the gene-trapped allele, and completely absent from homozygous cells and tissues.