CBS and Dravet syndrome: However, systemic and chronic overexpression of CBS due to additional copy of CBS gene in trisomy 21 (Down syndrome, DS) elevates the cellular levels of H2S to toxic levels, which (on the background of a variety of additional biochemical misalignments caused by additional gene triplications in this condition) suppresses mitochondrial Complex IV activity and impairs mitochondrial oxygen consumption and aerobic ATP generation [12].