A gene trap vector inserted into intron 1. The vector comprises an En2 intron, a lox71 site, an En2 splice acceptor site, the beta-geo (lacZ reporter - neomycin resistance) fusion gene with CpG island deleted, a poly(A) signal, an FRT site, a loxP site, a second poly(A) signal, a second FRT site, a lox2272 site and a lox511 site. (J:241325)