"Although enhancer traps are widely used, certain gene traps were developed as an alternative to enhancer traps to capture open reading frame information. The identification of target genes using enhancer trapping was sometimes problematic because the site of reporter insertion could be as much as 100 kb from the target gene. This would require extensive characterization of the genomic insertion site to identify candidate target genes. Gene trapping varies from enhancer trapping in that, instead of using a minimal promoter, gene trap vectors provide specific sequences that generate fusion RNA transcripts when inserted into a gene (See Fig. 1). This feature makes gene trapping (vs. enhancer trapping) especially advantageous in mammalian cells that have complex genomic organization, including large introns and small exons, because the trapped gene can be identified by mRNA sequence. "
From: [Durick K, et al. “Hunting with traps” Genome Research 9(11): 1019-1025. Nov. 1999] - nice review article.