Co-expression of multiple genes is valuable in many experimental settings. To achieve this, scientists use a multitude of techniques including co-transfection of two or more plasmids, the use of promoters, or the creation of polycistronic vectors.
Detecting cells that are expressing your gene, especially if you are studying a novel gene, is not always a straightforward process. Polycistronic plasmids ensure that any cells that are positive for your marker should also be expressing your gene as they are both derived from the same transcript.
Polycistronic vectors are useful almost anytime you want to express multiple genes in the same cell. In sequencing we treat these plasmids as any other regular plasmid. However, when designing primers for sequencing we recommend that customers do not use primers that have several annealing regions. Instead, use more primers as each sequencing reaction typically produces 700bp of usable data.