Methylation is a biochemical process where a methyl group, consisting of one carbon atom and three hydrogen atoms (–CH₃), is added to a molecule, often DNA. This modification can influence gene expression without changing the DNA sequence itself, playing a crucial role in regulating various biological functions. In the context of genetics, methylation can affect how genes are turned on or off, impacting development, aging, and disease susceptibility. Abnormal methylation patterns have been linked to conditions such as cancer and autoimmune diseases, highlighting its importance in health and disease management.