A gene trap vector, containing the splice acceptor site, beta-gal, and hygromycin phosphotransferase gene (hygR), was randomly inserted into the first intron of the gene, creating the 4B8 mutant ES cell line. RT-PCR analyses indicated that RNA transcripts were not detected in E10.5 mutant embryos. Western blot analysis showed that the protein was absent in mutant retina. (J:157907)