Antisera are important tools for studying gene products - proteins

Once a putative identity has been established by sequence comparisons, it is possible to purify the corresponding protein to homogeneity and prepare an antiserum. As an alternative, the cDNA can be expressed in E.coli  as a translational fusion and the expressed protein used to prepare the antiserum. This is normally done as fusion constructs with e.g. -galactosidase (LacZ), histidine tags (purified using nickel affinity media) or glutathione-S-transferase fusions.

A translational fusion of a cDNA with the E.coli lacZ protein in a vector carrying a drug resistance marker (AmpR for ampicillin) and origin of replication (ori). In a translational fusion, the two parts - your gene and the marker - are put together to make a single transcript which is processed to make a single protein.

It is possible to make a cDNA library in such a vector in order to screen with an antiserum or even an enzyme activity.