hijacking viruses

many diseases are characterized by the lack of something, perhaps an enzyme. or sometimes it is benificial to provide a growth factor. in these cases, it would be benificial to add a gene to a cells DNA. but how? well, some really smart people realized thats what viruses do, so why not hijack them and make them work for us? so they did. there are 5 kinds of viruses we use.
1)adenovirus- most commonly used; infects respiratory epithelium,
disadvantages- very immunogenic (causes immune reaction), infection only lasts a few months
2)adeno-associated virus - infects many cell types, no immune response
disadvantage- hard to harvest to any large number
3)retrovirus - used to treat immune deficiencies, generated in high titer
disadvantage- only integrates into dividing cells, and the site of intigration is unpredictable so it could insert anywhere and cause cancer!
4) lentivirus - virus associated with HIV, infects all cells, gene expression lasts for years
disadvantage - site of integration unpredictable
5) herpesvirus - (yes, as in herpes the STD), infects neurons
disadv. - large and dificult to use

once you insert a gene via a virus, it is always there, you can never get it out. so, hope you don't screw up! what they started doing is inserting a progene, meaning you have to give a drug or something to make that gene active. when you don't want the gene to be expressed anymore, then you stop taking the drug. kind of eliminates the purpose of inserting the gene in the first place (so you don't have to remember to take your drug) but it still has its advantages.