We are very comfortable with the concept that variations in our genetic make-up contribute to our phenotypic differences. Differences in the genetic sequence between two individuals are what help to make them unalike. Alleles are two or more functioning forms of any one particular gene, distinguished from one another by differences in DNA sequence. The inheritance of different alleles of the same gene may result in phenotypic differences. Sections of DNA may also be repeated a number of times, and this number may vary between individuals. These ‘copy number variations’ may also lead to phenotypic differences. We also recognize that these DNA sequence differences are inherited by daughter cells, during cell division, and that they are mostly irreversible. As cells age, divide and are exposed to the environment, changes in the underlying DNA sequence (mutations) may occur and alter gene function. In some cases, these changes may cause disease, such as cancer. The degree of variation in DNA sequence between two individuals, and even between two different species, is actually quite small, and yet phenotypic differences may be vast. Furthermore, despite all the cells in any one person essentially having the same genetic sequence, different cell types display such varied functions and abilities (i.e. phenotypes). A cardiac cell is clearly very different to a neuron. These differences cannot be wholly explained by differences in the genetic sequence. The explanation, of course, lies with gene expression, and how it varies between cell types, and between individuals. Epigenetics describes inheritable variations in gene expression (between cell types, and between individuals) that result from differences other than DNA sequence variations. Epigenetics is all about what switches some genes on, and others off. Although the mechanisms are complex, in simple terms they all involve the binding of chemical tags directly to DNA, or to messenger RNA, which then influence how a gene is read, or how its messenger RNA product is used. So, the underlying DNA sequence may be the same, but if a gene is silenced in one cell type, but not another, then the cell phenotypes will be different.