The mouse embryonic stem cell line RRB047 (strain 129/Ola) containing a gene trap insertion in Trip13 was obtained from BayGenomics (http://www.baygenomics.ucsf.edu/). The gene-trapping vector used to create this line, pGT1lxf, was designed to create an in-frame fusion between the 5′ exons of the trapped gene and a reporter, βgeo (a fusion of β-galactosidase and neomycin phosphotransferase II). The gene-trapped locus creates a fusion transcript containing exons 1–3 of Trip13 and βgeo. To identify the exact insertion site within intron 3, PCR was performed using one primer within the gene trap vector, and the other primer at various positions in intron 3 pointing towards the 3′ end of the gene. Product from a productive reaction was sequenced, revealing that the insertion site was 445 bp into intron 3.