What are the causes of DMD? DMD is caused by a defect in one of the genes in the body. Doctors call this a mutation. Our body is made up of trillions of cells. Virtually every one of these has a control centre (a nucleus), which contains genes. These genes contain the instructions our body needs to develop, live and reproduce. The largest gene in our bodies contains the instructions for making a protein called dystrophin. Dystrophin is found in all muscles in our bodies and plays an important role in protecting our muscles whilst they are working. All the genes in our bodies are packaged into ‘groups’ called chromosomes. These chromosomes are inherited from our father and mother. If you are a girl, you have two X chromosomes (one X from your mother and one X from your father). If you are a boy, you have one X chromosome (from your mother) and one Y chromosome (from your father). The dystrophin gene is only located on X chromosomes. As boys only have one X chromosome, DMD is much more common amongst boys. In two thirds of cases, the faulty dystrophin gene is passed from the mother to the child. These women are called ‘carriers’ because they carry the DMD gene although they do not have the condition themselves. In one third of cases, a spontaneous mutation happens at the beginning of the pregnancy. No one is to blame for either inherited genes or spontaneous mutations. There is nothing that anyone could have done before, during or after the pregnancy, to prevent a child from developing DMD. Genes are very complicated and sometimes they go wrong.