A gene trap vector (pGT1Lxf) containing a splice acceptor, lacZ and neomycin resistance gene was inserted into intron 13. RT-PCR showed strong expression upstream of the inserted cassette, however, expression downstream of exon 13 was not detected in mutants. Slight traces of normally spliced exon 13-14 were detected in mutants, suggesting the disruption might have resulted in a hypomorphic allele.