Introduction

To correctly read the information stored in our DNA genomes (the genetic code), cells must read another language that overlays it, the epigenetic code, which controls access to that information. A process such as transcription can only retrieve this information according to the access granted by the epigenome. The term epigenetics was coined in the 1940s by British embryologist and geneticist Conrad Waddington to describe "the interaction of genes with their environment, which bring the phenotype into being". Now the term epigenetics (literally over or above genetics) refers to the extra layers of instructions that influence gene activity without altering the DNA sequence. There are three main components to the epigenetic code: (i) methylated cytosine residues in DNA1; (ii) the range of post-translational modifications to the core histone proteins within the nucleosomes (referred to as the histone code)2,3; and (iii) RNA molecules, often non-coding RNA4. © 2010 The Biochemical Society.