Abstract: The invention provides compositions and methods for determining whether a subject is predisposed to the disease or condition, or for diagnosing a disease or condition, or for detecting the state of a disease or condition, by detecting the methylation state of the subject's nucleic acids. In addition, the invention provides methods for determining the methylation age of a subject or tissue from a subject or for differentiation between nucleic acids originating from different subjects or tissues. The invention further provides methods for selecting nucleic acid molecules for use in the methods of the invention.