Unpicking the grammar of genes

The MHC on the short arm of chromosome 6 is the most gene-dense region of our DNA with around 230 genes all crammed into this stretch of our genome. The MHC , or major histocompatibility complex, is known to play a pivotal role in our immune system, and around a third of the genes encoded there are known to have immune functions (the functions of all the genes are not known as yet, so it could be more). So it's not surprising that DNA variations in this region have been linked to many autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis, and coeliac disease. But the MHC has also been linked to diseases not related to the immune system, including breast cancer, asthma, infectious diseases and the adverse effects of certain drugs. It's the genetic region with the largest number of diseases associations, period. But finding a genetic link to a condition is one thing. Determining the specific DNA changes that cause the increased risk of disease is another.