A new strategy of gene trapping in ES cells using 3'RACE.

"Gene trapping" in embryonic stem (ES) cells is a novel approach to identify a series of genes in mammals concomitant with the production of the corresponding mutant mice. However, this approach is currently unable to identify genes that are not expressed in ES cells. Here we describe a strategy to identify gene trapping clones which is not based on expression of a reporter gene. It uses the neor gene which lacks a polyadenylation signal and has a splice donor signal. Expression of the neor gene as fusion transcripts with the 3' end containing the polyadenylation signal of tagged genes allows the identification of these clones by 3' rapid amplification of the cDNA end in undifferentiated ES cells, even if the genes are not expressed in ES cells. Amplification was observed in about 25% of G418-resistant clones. Sequence analyses suggested the amplifications represent gene trapping events. The feasibility of this approach was further assessed by analysing one clone, PAT-12, in detail.