What is the difference between these three terms: Epigenetic tag, Epigenetic process, Epigenetic mechanism?
Response written by: Elphège Nora, PhD – postdoctoral researcher in Edith Heard's laboratory, Institut Curie
Strictly speaking, the word “epigenetic” characterizes the mode of inheritance of a given trait, irrespective of the molecular mechanism or process that underlies it. “Epigenetic process” and “epigenetic mechanisms” are synonymous expressions often employed to refer to the molecular process or mechanism that underlies the epigenetic inheritance of a given trait. Now you understand that in order to be perfectly correct one should actually not use the terms “epigenetic process” or “epigenetic mechanism”, as strictly speaking it is not the process or mechanism that is epigenetic in essence. “Epigenetic” is instead an adjective characterizing of the mode of inheritance. However “epigenetic process/mechanisms” are convenient words and widely used as it is much easier to talk about your favorite “epigenetic process” that your favorite “process that underlies the epigenetic inheritance of your favorite trait”! But really that big sentence is what one means by “epigenetic process”.
Now “Epigenetic tag” is tricky. According to what I just said, what does “epigenetic tag” actually mean then? Is it a molecular tag that is involved in the epigenetic inheritance of a given trait, or is it a tag that is epigenetically inherited? That really is not the same thing! In fact I have met people discussing epigenetic tags (or “Epigenetic marks” in that case), who were arguing and thought they were disagreeing but in the end they realized they were using the same word to talk about two different things. In these conditions the best to avoid confusion is to always remember that “epigenetic” is an adjective qualifying the mode of inheritance, not a molecule, a tag or a mark.