system
Given the following post, respond as if you're commenting on Reddit:
human
Can anyone explain to me why the CRISPR/Cas system is supposed to be a huge leap forward in gene editing? Why is it better than using viruses to insert genes?

I think transgenics is a cool field and I'm trying to add to my knowledge, what do you guys think?
gpt
CRISPR/Cas9 is indeed a giant leap forward in gene editing technology. It should be noted that it isn't "better" than using viruses, in fact, we often use viruses to deliver the CRISPR system to our cells of interest. 

CRISPR is amazing for a number of reasons. The Cas9 enzyme makes double stranded DNA breaks in a targeted fashion: you can supply the enzyme with a short guide RNA (sgRNA) that directs Cas9 to a specific site in the genome, where it does it's thing. In the simplest use of CRISPR/Cas, this alone can break your gene of interest. Most of the time, homologous recombination will repair the break just fine, but this means the the sgRNA target sequence is still present, so Cas9 will hit it again. At some point, the DSB will be repaired by non-homologus end joining, the gene will likely be broken, and the sgRNA target sequence will be gone, so this is a stable end point. 

For more nuanced genome editing, you do the same thing as above, but you supply an alternate repair template, with a particular mutation of interest included. At some frequency, now your DSBs will be repaired off of your mutant template, and in this manner, you can quickly introduce just about any mutation that you want, at nearly any location in the genome. Critically, we've had genome editing capabilities for a long time, but CRISPR/Cas make it **much much** more efficient. It used to take up to a year or more to design and make a targeting construct, get it into cells of interest, and select for targeted events. This was generally reserved for making targeted mutations in ES cells that would go on to become transgenic mice. With CRISPR/Cas, because it's so much easier and quicker, now people are editing all sorts of cancer cell lines, and are able to ask questions that we previously had been unable to answer.