We can engineer viruses because we understand them, they are not very complex. They have genetic information in their genomes and then they have proteins that package the genetic information. So, many viruses, we are able introduce the genes that we want into the virus genome and the virus can then carry those genes into particular cells that they are able to target. So, by this way we can actually introduce new genes into targeted cells where those cells then, once infected with the virus, will produce the protein that we've introduced into the virus. So, in such a way we can fight cancers, we can use it to actually deliver a system that's called gene-editing system, that's recently being harnessed to fix changes, or fix mistakes or make changes into the genome of the organism itself. So, viruses are incredibly simple in one respect, and very useful to deliver these types of materials into the cells of choice.