The pGt1lxf gene trap vector, which integrated randomly into the first intron - which interrupts the coding sequence within the tenth codon - contains a splice acceptor sequence upstream of a Beta-geo (beta-galactosidase/neomycin phosphotransferase II fusion) reporter gene. Transcription from the endogenous promoter of the trapped gene and splicing of the message yields a fusion mRNA joining exogenous upstream exons to the Beta-geo coding sequence. (J:94077)