Figure 1. Mechanism of gene repair. Chimeraplast and coiled DNA must first pair. This is promoted by the RNA–DNA annealing, creating the mismatch with both strands of the chromosome, where X and Y are complementary base-pairs. After pairing is complete, the mismatch repair occurs, producing changes in both strands of the chromosome. many years to identify genes whose expression patterns change upon T cell activation. Moreover, the approach identified two genes with an unprecedented pattern of response to stimulation: Their expression was upregulated in response to stimulation with either phorbol ester or thapsigargin, but downregulated by stimulation with both agents. Such genes are of considerable interest, since various functional states of T cells, notably anergy and peripheral tolerance, are also selectively manifest in conditions of incomplete stimulation or partial activation8. Unfortunately, it is not currently feasible to generate the libraries in primary T cells, where target genes of greatest relevance to anergy and tolerance are likely to be expressed.

The requirements and applications of the Whitney et al. method are remarkably complementary to those of DNA microarray technology, another powerful method for monitoring changes in gene expression9, 10. The microarray approach can be used with essentially any cell source including complex tissues, whereas the Whitney et al. method is currently limited to proliferating cells from which gene-trap libraries can be generated. The microarray approach samples a fraction of all expressed genes, those represented in the DNA sequences used to generate the microarrays, whereas the Whitney et al. method could potentially sample the entire genome if large enough libraries (containing approximately 2× 107 independent integrations) could be generated. From the biotechnology point of view, a particular advantage of the Whitney et al. method is its applicability to drug discovery in living cells. Cloned cell lines in which reporter expression is controlled by specific signaling pathways can readily be generated and used in high-throughput screens to identify compounds that might inhibit or potentiate the response. Again, an important issue is whether the method can be adapted to study a sufficiently diverse range of cell types and tissues; nevertheless, its application even to existing cell lines promises to yield much new and important information.