A gene trap vector (pGT0lxf) consisting of an En2 splicing acceptor sequence (SA), beta-geo (lacZ fused with neomycin transphosphorylase fusion gene) and SV40 poly(A) site (pA), was inserted into intron 2. Western blot analysis confirmed the absence of protein expression in the small intestine of homozygous mutant mice, suggesting that this likely a null allele.