A retroviral gene trap vector, containing a 5' LTR followed by a splice acceptor, beta-geo, PGK-puro, stop codons, a splice donor, and a 3' LTR, was inserted into intron 2. Insertion of the gene trap vector led to deletion of the second RNA recognition motif and the four K-homology modules, known to be essential for RNA binding, cytoplasmic granule formation, and subcytoplasmic localization. Normal transcript was undetected by RT-PCR analysis of homozygous mutant embryos.