Retroactive genetic engineering is a currently non-existent form of genetic engineering that offers even more (hypothetical) potential than current forms.
Current genetic engineering requires that the alteration of the subject's DNA occur at the time of its conception. This requirement makes modern genetic engineering pratically useless for curing genetically transmitted diseases because couples rarely have their genetics examined before they copulate.
On the other hand, retroactive genetic enineering alters the subject's DNA after conseption, possibly into adulthood. The way this could possibly be accomplished is through the use of a DNA-altering virus. Some virii do more than just inject their DNA or RNA into their victim, but actually alter their victim's permanant genetic structure. Such a virus, stripped of its harmful effects, could be used to insert the modified DNA into the subject.
Of course, it's never as simple as it sounds. The first complication is the subect's own immune system. To allow the virus to alter the DNA of subject, some sort of immune system suppressant whould be required; this could introduce additional complications if the subject comes down with another virus (or other disease) while under the suppressant's influence.
The other major problem is that once you've changed the subject's DNA, the subject won't just suddenly change. For the changes to come into effect, the subject has to grow or develop in the area modified. If the subject is young and the changes minor, it may be possible just to wait, but for more global changes, that won't work. For signifigant changes or alteration of adult organisms, a number of growth and development hormones whould be required; however, it is very likely that those hormones whould tend to encourage cancer. If the changes are really radical, then the introduction of stem cells whould probably be nessessary to allow the production of new cell types.
Retroactive genetic engineering hasn't been achived, and is perhaps a long way off, but when it happens, modern medicine will be drastically changed.