The retrovirus gene trap vector GTR1.3 containing a puromycin gene, a lacZ gene, a Pol2 gene and a neo cassette was inserted into intron 3. The absence of protein product was confirmed by western blot analysis on embryonic fibroblast extracts. Northern blot analysis confirmed the expression of a transcript of increased size in embryonic fibroblasts. (J:137415, J:137511)