Conrad Waddington introduced the term epigenetics as the “branch of biology which studies the causal interactions between genes and their products, which bring the phenotype into being.” Over recent years epigenetic processes have been defined in many ways, from “a change in the state of expression of a gene that does not involve a mutation, but that is nevertheless inherited in the absence of the signal (or event) that initiated the change”1 to a much broader definition: “the structural adaptation of chromosomal regions so as to register, signal or perpetuate altered activity states.”2 The lack of a clear, agreed definition means the term epigenetic is now used to describe many processes associated with the control of gene expression, including DNA methylation, histone modifications, and noncoding RNA, despite the fact that we do not know of biochemical mechanisms allowing histone modifications or noncoding RNA to be “heritable” from parent to daughter cell through mitosis or meiosis. Here we focus on the most widely studied “epigenetic” modification, DNA methylation, which occurs predominantly at cytosines in a CpG dinucleotide context.
Greally JM, Drake AJ. The Current State of Epigenetic Research in Humans: Promise and Reality. JAMA Pediatr. 2017;171(2):103–104. doi:10.1001/jamapediatrics.2016.3508
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