"In contrast to the rational "one gene at a time" approaches, enhancer-trapping methods evolved to probe the entire genome simultaneously. Originally described in bacteria, enhancer trapping was first demonstrated using bacteriophage transposable elements to insert a reporter gene at scattered sites throughout the E. coli genome (Casadaban and Cohen 1979; Bellofatto et al. 1984). Chromosomal integration of a transposable element tagged the integration site and often mutated the gene into which it inserted. Mutants resulting from this approach could be selected using genetic screens. Because the insertion site was tagged, the mutated genes could be readily identified, which made this approach more efficient than traditional chemical mutagenesis (Meneely and Herman 1979; Rinchik 1991). The success and conceptual simplicity of the enhancer trap method was quickly adapted for use in other model systems including plants (Schell 1987), C. elegans (Hope 1991), and Drosophila (O'Kane and Gehring 1987). When the enhancer-trapping element consists of a cDNA encoding an easily monitored reporter gene like -galactosidase, the expression pattern of the trapped gene can be visualized. In Drosophila, the P-transposon system has been used with great success to create transformed animals carrying enhancer detectors (Rubin and Spradling 1982; O'Kane and Gehring 1987). Large numbers of enhancer trap lines have been established and evaluated using both phenotypic and expression analysis (Bier et al. 1989; Spradling et al. 1995). Specific mutant lines can then be chosen for further study when there is a correlation between expression and phenotype. " From: [Durick K, et al. “Hunting with traps” Genome Research 9(11): 1019-1025. Nov. 1999]