Complex sentence: One possible solution for this is to add a functional tumor suppressor gene onto the DNA to be integrated; however, this poses its own problems, since the longer the DNA is, the harder it is to integrate it efficiently into cell genomes.
Split into simpler sentences: One possible solution is to add a functional tumor suppressor gene to the DNA to be integrated. This may be problematic since the longer the DNA is, the harder it is to integrate into cell genomes.