 Understanding what an epigenetic clock is first requires understanding epigenetics. In a nutshell, epigenetics refers to structures and modifications that affect how genes are expressed without changing the DNA sequence itself. For now, you just need to know about a type of epigenetic change known as DNA methylation. Methylation is the attachment of a molecule known as a methyl to DNA. The presence or absence of methyl affects how active a gene is. The methylation pattern throughout the genome changes in response to various factors, including age. That's the basic idea behind an epigenetic clock, measuring the amount of methylation at various locations in the genome to estimate age.