How to genetically alter a rodent...

A couple of weeks ago I had a conversation with a girl at a party. Her job is to alter the DNA of mice in order to cure diceases. That completely blew my mind. That sounded SO exiting, but she excused for her boring job and went out to dance.
As I later called up one of my genetic science-friends (Yes, I have one of those) who aught to know about stuff like this, the only response I got was: "You cant do it on your own - Forget it!"
I could probably google my way into a correct answer, but instead I have chosen to guess. This is my full report on mice and DNA. I did it while waiting for a renderer at work today. It probably wont get the Nobel price. Neither in science nor litterature. But it was better than just waiting...