Epigenetics is the study of heritable changes in gene expression that do not require, or do not generally involve, changes in the genomic DNA sequence. Historically, the term referred mainly to developmental phenomena, but recently the term has been applied more broadly to signify a relation to gene action. Currently, the field is primarily concerned with understanding the handling of genetic information by eukaryotic cells.

Genetic inheritance has been regarded until recently as the sole mode of transmission of information from one generation to the next, but a number of challenges remain in understanding the transmission of genetic information and gene expression despite the successes surrounding the unveiling of the human genome.1,2 Epigenetic inheritance, which implies modification of gene expression without modifying the DNA sequence, has been proposed as a mechanism complementing genetic inheritance to explain these phenomena. Epigenetic information is transmitted by way of direct modification of DNA or of chromatin. In mammals, DNA methylation of cytosines is the only known physiological modification of DNA, whereas numerous modifications of chromatin have been identified that affect its conformation, but they have proven much more difficult to sort out and their significance is as yet only partially understood.

Many aspects of epigenetics have been examined in abundant detail, particularly within the past 5 years. Although PubMed searches (December 6, 2006) under ''epigenetics'' for the years 2000-2006 turned up only 533 citations of all kinds including 242 reviews with 197 reviews on humans, it seems apparent that many more epigenetics papers are in the literature, particularly in the older literature before many studies were identified with the term epigenetics. In assembling material for inclusion, an attempt was made to highlight conceptual advances and focus on key steps along the pathway to epigenetics. It is obvious from a timeline of epigenetic research constructed from those PubMed searches that the scope and pace of the investigation changed increasingly over time, particularly since the mid-1970s when a number of new techniques for monitoring the biochemical nature of epigenetic change were developed.

This chapter was written with the purpose of summarizing the emergence, foundations, and status of epigenetics by assembling conceptual approaches, analytical tools, and experimental evidence that led to the emergence of epigenetics as a field of biological inquiry primarily concerned with understanding the handling of genetic information by eukaryotic cells. It concludes with some perspectives on prospects for the field in health and disease.

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