Methyltransferase is an enzyme that transfers a methyl group (–CH₃) from a donor molecule, usually S-adenosylmethionine (SAM), to specific substrates, such as DNA, RNA, or proteins. This process is crucial for various biological functions, including gene regulation, protein function, and cellular signaling. These enzymes play a significant role in epigenetics, where they can modify DNA and affect gene expression without altering the underlying genetic code. Methylation can influence processes like development, differentiation, and response to environmental changes, making methyltransferases essential for maintaining cellular health and function.