Trapped gene association determined by genome coordinate overlap and strand. Gene-trap construct pU-21B was randomly inserted in intron 2. The construct pU-21B consists of 1.8 kb of an intron and a splice acceptor sequence from the mouse En-2 gene, a promoterless beta-geo gene containing three stop codons in-frame with the ATG, and a polyadenylation signal. (J:212073)