As described above, the IRES-mediated polycistronic expression system possesses some weak points. One of them is that protein accumulation from the 5′ proximal
gene might be lower than when it is expressed alone. This negative effect sometimes may limit opportunity to use the IRES system. Most possible usage is that we check
expression level of a transgene in stable transformants by IRES-mediated co-expression of a reporter gene. In such cases, the 5′ proximal transgenes are limited to those of
which functions are active independent of amount of the products.