When you look at yourself in the mirror you may ask, ‘How, given that all the cells in my body carry the same DNA, can my organs look so unlike and function so differently?’ With the recent progress in epigenetics, we are beginning to understand. We now know that cells use their genetic material in different ways: genes are switched on and off, resulting in the astonishing level of differentiation within our bodies.

Epigenetics describes the cellular processes that determine whether a certain gene will be transcribed and translated into its corresponding protein. The message can be conveyed through small and reversible chemical modifications to chromatin (figure 1). For example, the addition of acetyl groups (acetylation) to DNA scaffold proteins (histones) enhances transcription. In contrast, the addition of methyl groups (methylation) to some regulatory regions of the DNA itself reduces gene transcription. These modifications, together with other regulatory mechanisms, are particularly important during development – when the exact timing of gene activation is crucial to ensure accurate cellular differentiation – but continue to have an effect into adulthood.