Gene trap (GT) strategies in mouse embryonic stem (ES) cells are increasingly being used for detecting patterns of gene expression (1-4, isolating and mutating endogenous genes (5-7), and identifying targets of signalling molecules and transcription factors (3, 8-10). The general term gene trap refers to the random integration of a reporter gene construct (called entrapment vector) (11, 12) into the genome such that ‘productive’ integration events bring the reporter gene under the transcriptional regulation of an endogenous gene. In some cases this also simultaneously generates an insertional mutation. Entrapment vectors were originally developed in bacteria (13), and applied in Drosophila to identify novel developmental genes and/or regulatory sequences (14-17). Subsequently, a modified strategy was developed for mouse in which the reporter gene mRNA becomes fused to an endogenous transcript. Such ‘gene trap’ vectors were initially used primarily as a tool to discover genes involved in development (1, 2,18). In the last five years there has been a significant shift of GT approaches in mouse to much broader, large scale applications in the context of the analysis of mammalian genomes and ‘functional genomics’. Sequencing and physical mapping of both the human and mouse genomes is expected to be completed within the next five years. Already, a large number of mouse and human genes have been identified as expressed sequence tags (ESTs), and very likely the majority of genes will be discovered as ESTs shortly. This vast sequence information contrasts with a rather limited understanding of the in vivo functions of these genes. Whereas DNA sequence can provide some indication of the potential functions of these genes and their products, their physiological roles in the organism have to be determined by mutational analysis. Thus, the sequencing effort of the human genome project has to be complemented by efficient functional analyses of the identified genes. One potentially powerful complementation to the efforts of the human genome project would be a strategy whereby large scale random mutagenesis in mouse is combined with the rapid identification of the mutated genes (6,7,19, and German gene trap consortium, W. W. unpublished data).
Genomic Medicine
