Most of us have heard of genes, but what are they exactly? A gene is a segment of DNA that contains the instructions for the production of biological molecules usually proteins. Humans have around 20,000 genes, all containing the information needed to build one or more proteins. Some genes determine physical characteristics such as the colour of our eyes. Others can influence our risk of developing certain conditions such as diabetes. But genes only account for around 2% of all adonetic information, which is made up of more than 3 billion letters of DNA called bases. While we don't yet understand the exact function of all the other 98%, we do know that changes to this DNA can also impact on our health. So where is all this DNA stored? Almost all of our DNA is arranged into tightly-coiled structures called chromosomes. Humans have 23 pairs of chromosomes, with one of each pair inherited from our father and the other from our mother. Chromosomes are located in the nucleus of a cell. A very small amount of DNA is also found in the mitochondria, a structure which provides energy for the cell. All the DNA contained in one cell is called the genome. We have one copy of our genome in nearly every cell in our body. Our genomes are approximately 99.8% identical to that of every other human being. It's the 0.2% variation in our genome that interests us in health care. As understanding it can help in the prediction, prevention, diagnosis and treatment of disease.