system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why can't scientists program viruses that kill cells with deformed DNA to cure cancer?
gpt
Short answer: They're trying, but it's hard. This will probably be how we eventually manage to "cure" cancer.

Long Answer: "Cancer" is a general term which generally refers to any unwanted/out of control growth of the body's own cells. As you correctly said, it's due to deformed/mis copied DNA, which shuts off the protein pathways which prevent abnormal growth (both by stopping cell division, and trigger apoptosis (cell death) in cells which shouldn't be somewhere). There are a couple of things which make this hard to treat by sticking a virus into the body.

Firstly, cancers are not composed of foreign cells and generally have the same markers on their surface as rest of the body's cells, so the immune system doesn't attack them. This isn't always the case, it is possible for the immune system to be "taught" to attack cancerous cells *in vitro* (outside the body). In fact this was recently done by a team using a modified version of the HIV virus and they had some success treating an individual's leukaemia. Viruses are a good approach because they hijack host cell's DNA to produce their own proteins/reproduce, so you can use them to add specific genes to cells. (this is how some GM crops are made). The problem with this is that we are still a huge way from being able to custom design a gene which will produce a protein with function X, so our ability to kill cancer cells using this approach is limited. The team using the HIV cells made some of the body's own immune cells recognise naturally occurring "signals" on the cancerous cells; but this was pretty specific to the type of cancer they were treating.

Secondly, each cancer is different. While a lot of cancers have mutations to certain genes which regulate cell growth, this doesn't necessarily mean that they "look" the same on the outside to immune cells, (especially as each individual has different immune antigens anyway) so a treatment would almost certainly have to be tailored to each individual which is prohibitively expensive (and basically impossible with current knowledge / computers). 

Hopefully once we get better at understanding how proteins fold, and hence what function they have, we'll be able to custom design virii which will attack cells with malformed DNA.