Cytochrome c oxidase activity following RNAi-mediated knockdown of MECP2. Cytochrome c oxidase (COX) activity was determined as a first order rate constant and expressed as rate/minute/mg protein, whilst the specific activity of the mitochondrial matrix enzyme citrate synthase (CS) was determined and expressed as nmol/minute/mg protein. COX was normalized to CS activity to take into consideration any variation in mitochondrial numbers, and is expressed as a ratio. Three days post-transfection, when MECP2 knockdown is greatest, cytochrome c oxidase activity is just 60% of that observed in the untreated sample at Day 0 (p = 0.005) and the control sample at Day 3 (p = 0.015). KD = knockdown.