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.
It often happens on promoters, where it inhibits transcription.
Incredible that there hasn't been a Nobel Prize for it as of 2022, e.g. as mentioned at:
The main way to sequence DNA methylation. Converts methylated cytosine to uracil, and then we can sequence those.
Video 1. Bisulfite Sequencing by Henrik's Lab (2020) Source. Nothing much new that we don't understand from a single sentence in the animation. But hey, animations!