TMEM126B is a mitochondrial protein that is involved in a very important energy exchange that happens within mitochondria – the tiny batteries of the cell. When TMEM126B is faulty, this energy exchange doesn’t work properly and the cell’s energy supply is interrupted. This causes major problems for how the cell works and causes disease. There are many different types of “mitochondrial disease” and that due to faulty TMEM126B is just one those. It is a very rare condition with only a few people described who have faults in TMEM126B, but they all had symptoms affecting their muscle development. The faulty TMEM126B protein means that the first and a very important step in the energy exchange process, known as complex I activity, is impaired. The TMEM126B gene faults are most often inherited, one from the mum and one from the dad. The parents have a 1 in 4 (25%) chance of having another affected child, for each pregnancy. Genetic counselling or referral to a clinical (mitochondrial) specialist is recommended for families affected with mitochondrial disease.