The first found and most important known epigenetic marker.
Happens only on adenine and cytosine. Adenine methylation is much less common in mammal than cytosine methylation, when people say "methylation" they often mean just cytosine methylation.
Incredible that there hasn't been a Nobel Prize for it as of 2022, e.g. as mentioned at: theconversation.com/no-nobel-but-epigenetics-finally-gets-the-recognition-it-deserves-18970
Some old dudes getting another prize in 2016: www.cuimc.columbia.edu/news/pioneers-epigenetics-awarded-horwitz-prize