Until recently modifying the DNA in people has been a mostly random process. Homologous recombination has been the main technique, where repair enzymes splice in DNA. This is a 1 in 100,000 chance event for mice embryonic sytem cells. It is less likely in other types of cells like human cells. The splicing occurs at random points. A second generation of transgenics techniques are being developed which will provide more control
New techniques have a 1 in 5 chance of inserting DNA. New techniques are allowing more precise control of where the DNA goes.
Site specific DNA insertion can be done using enzyme recombinases (what viruses do to bacteria You get a specific target DNA taken up into an animal that you want to genetically modify. You wait until one of the animals gets it in the right location. Then you breed that animal. You can then add new DNA at the right spot. This is like randomly get a tape cassette in the right spot and being able to put in different cassettes to play.
Michele Calos of Stanford University has used bacteriophage phiC31 to be able to splice in DNA This can be done at 101 sites (19 main sites and 82 less likely). There was still no control over which one of those sites that the DNA you wanted to splice in went. 101 sites is still better than completely random, where the DNA could go to unsafe spots.
By modifying the phiC31, specific sites can be targeted
There is also the creation of molecular scissors using zinc fingers and DNA cutting enzyme nuclease.
Sangamo Biosciences of Richmond, CA has created custom zinc fingers which can target specific locations
The modified zinc fingers have been used to make human cells resistant to HIV This will be going to first stage clinical trials in the second half of 2006.
Modifying recombinase and modified zinc fingers could allow DNA to be inserted exactly where you want in people.