Methyl

Methyl is an ion. It is made by removing one of the hydrogen atoms from an atom of methane. It is an organic ion. Its chemical formula is CH3. It bonds with other ions, such as hydroxide, to make chemical compounds. The bonding with hydroxide makes methanol. If it bonds with mercury, it makes dimethylmercury.
Methyl groups added to cytosine are the basis of epigenetics. They change the way genes act, and so explain how (for example) identical twins may turn out differently later in life. Or, they help understand how a mother's nutrition may have long-lasting effects on her offspring.