In the course of analyzing a regulated gene or genes, it is often useful to monitor the regulatory system by putting a foreign gene under the control of that system. For the activity of the reporter to reflect the regulatory scheme of interest, the foreign gene needs to be separated from its own regulatory signals and placed transcriptionally downstream from the regulatory sites under study. Such a merger of the two different regions is termed a fusion, and is used when the regulation of the system is the important topic. The inverse case, where a gene of interest is placed downstream of a strong or tunable promoter, is typically called an expression vector and is employed when the function of the gene product is the critical issue.
Fusions fall into two classes that require separate definitions.
This allows you to monitor transcriptional regulation of the target gene and ignores any possible posttranscriptional effects. Further, the reporter protein must be functional even when synthesized as a fusion protein. However, introduction into that transcript of a gene with an easily selectable or screenable gene product can allow the production of desired mutants because the regulatory system is now regulating a more technically addressable gene and gene product. That is, you want to see the effect of your new regulatory mutation in an otherwise wild-type background and determine how it affects the norally controlled genes, not merely the reporter. Thus, antibody directed toward the product of the introduced gene will allow the isolation of a protein with some portion of gene B product which in turn can be used to raise B-specific antibodies.