Complex I activity assay. Complex I activity measurement was performed 6, 12, and 24 h and 6, 12, 24, and 48 h after apoptosis and differentiation induction, respectively. 48 h after apoptosis induction, almost all of the cells were dead, and no complex I activity was observed. (The data are expressed relative to control). The data are means ± S.D. (error bars) of triplicate experiments.