From Lifespan – Why We Age and Why We Don't Have To" lecture on Talks at Google

Epigenetic – is any process or structure that govern the way how genetic information is packaged and read by the cell. Green parts on the picture – they spool up the genome like a you would a garden hose. Wherever the genome is wrapped this are genes that are switched off. Wherever the part is unwrapped this is the part will be switched on.