Sterols are a type of lipid (fat), the most common being cholesterol. These have a number of functions in the body including allowing normal brain development, particularly of myelin – the insulating sheaths which cover the nerve cells in the brain. Steroid hormones and bile acids are also derived from cholesterol. A defect in an enzyme which is involved in sterol metabolism, can result in problems affecting the development of the nervous system, leading to abnormal brain function. Behavioural problems are also common in these disorders as are accumulations of fats, and derivatives such as bile acids.