Sometimes referred to as a "second genome," scientists are studying genetic markers that appear to regulate gene expression and may be one key to curing chronic diseases.
While we all inherit a set of genes that determine our makeup, epigenetics, in contrast, studies how these gene markers are created, change in response to stress and the physical environment and can, quite literally, be passed from generation to generation. Scientists speaking on the PBS NOVA series on the subject described how this "second genome" could explain, for example, why identical twins, who have the same exact genetic makeup, ultimately wind up with completely different health histories.
An epigenetic change essentially alters phenotype without changing genotype.
That distinction goes to the success of some new epigentic cancer therapies being tested, which are much more targeted than current therapies. For example, one oncologist appearing on the broadcast said that the new methods treat cancer, not by killing the cell, but "through diplomacy," by reminding the genes responsible for cell division that this "is a human cell" and "it shouldn't be behaving like this." The quote had me grabbing the nearest pencil and paper so that I wouldn't forget.