Since then, there have been many studies of people with LHON and their families, and we now know that there are at least 18 different changes in genes linked to LHON, but only 3 of these changes account for almost all the known cases. The other 15 gene changes are found in people who also have one of the 3 main changes. The 3 main changes are called the primary LHON mutations. This is a complicated chemical, but can be thought of as an incredibly long, thin structure that, when magnified, looks a bit like a twisted ladder. Put simiply, each gene is a rung on the ladder.
Scientists can count along the 'ladder' and identify each gene by the number of the 'rung'.
This numbering system is not the only way of naming genes. Some people name genes by their purpose in the body, or the name of the chemical which the gene tells the body to produce. In this website I will tend to stick to the numbering system mentioned above, but in scientific papers the LHON genes are sometimes given other names. They are still talking about the same genes. The LHON genes are known to be the blueprint for part of a chemical called NADH Dehydrogenase or ND for short.