Check this out:
The video is divided into two parts. The second method (which begins 50 seconds into the second half of the video) is the Nobel Prize-winning technology of gene targeting.
This video comes from the Howard Hughes Medical Institute, the best thing Howard Hughes ever did. They support important research and also make some great stuff available (for free) to anyone who wants it: a magazine, videos, and a very cool series of CD-ROMs that let you pretend you're working in a lab, without the expensive supplies, toxic chemicals, or aching back.