Cytochrome c oxidase activity. 2 × 104 HT 29 cells were treated with 10 μg/ml of doxycycline over a period of 4 days and staining for cytochrome c oxidase activity was performed as mentioned in methods. The brown staining is indicative of cytochrome c oxidase activity, while purple staining is indicative of cell nucleus. Representative micrographs of six independent experiments are shown. Scale bar equals 20 μM.