Usually, the researcher focuses on a given gene, or group of genes, because of structural features that are perceived interesting. The two approaches have pros and drawbacks. Reverse genetics has come of age because of the genome project, which has provided the sequence of all genes. However, reverse genetics is risky because one may find no phenotype. Unfortunately for the scientists, but probably fortunately for the organisms, many genes can be inactivated without seeing an effect on the phenotype.
Transposons are important tools for both forward and reverse genetics.
In a forward genetics application, transposons are used as random mutagens. Transposons are DNA elements that have the ability to move from one chromosomal site to another. When they insert in a new site, transposons can act as mutagens if they insert into a gene. The expression of this gene may be reduced but not eliminated if the transposon inserted in a non-essential region. Often, however, the function of the gene is completely lost, such as when a transposon disrupted the coding region.