Respiratory chain complex activities were measured spectrophotometrically and normalized to citrate synthase activity in heart mitochondria from 8-week-old control and Tefm knockout mice (n= 5 mice for each group). The analyzed enzyme activities are NADH coenzyme Q reductase (complex I), NADH cytochrome c reductase (complex I/III), succinate dehydrogenase (complex II) and cytochrome c oxidase (complex IV). 