In medical research, what is the relative value of confirming old work vs. new discoveries? I am an MS student in medical immunology. I am looking for my research proposal, and would like to get a nice publication (i.e., publish in a journal with a high impact factor). Given this, is it better to do something totally new, or is there value in repeating old work?

I ask because my supervisor is busy and gives me considerable freedom. I have the idea of experimenting with the role of some gene in an autoimmune disease but no one has done that work before. Is it good to pursue this? When I write it up, how will I do citations?
Note, I have a physical/computer sciences background; things may be different in life sciences. 


  I have the idea of experimenting with the role of some gene in an autoimmune disease but no one has done that work before. Is it good to pursue this? 


Yes! Doing novel things will get more attention than revisiting old things (though both have value). 

My concern here would be whether you, as a lone, beginning student, have the background and resources to do something completely novel. Normally, one's first study is done under the close supervision of a mentor. I'm a little surprised your supervisor is "turning you loose" like this.


  When I write it up, how will I do citations?


Presumably parts of the methods you used have been used before (perhaps for other diseases), and someone has studied this disease before. So, you should cite any past work, even if it's not a direct hit to what you're doing.

But, this is sort of what I meant above -- it's good to have a mentor to help walk you through this process for the first time (or first several times) -- getting novel, publishable results with no supervision and little experience seems like a very tall order.