A gene trap vector consisting of a splicing acceptor site of the mouse Fgf4; an oligomer with three different frame termination codons; an IRES; Geo, which is the fusion gene of E. coli LacZ gene and neomycin resistance gene and an SV40 early mRNA polyadenylation signal was inserted in the middle of the 3'-most exon (3705-3723). The insertion also produced a 19 base deletion. (J:218057)