Transmission electron micrographs of H1299 and B9 (MtFt-expressing) tumors. No mitochondrial iron accumulation was observed in H1299 cell–derived tumors (A), whereas conspicuous iron deposits were detected in mitochondria of B9 cell–derived tumors (B). Oral administration of doxycycline to mice bearing B9 cell–derived tumors virtually abrogated mitochondrial iron accumulation (C-D). Representative transmission electron micrographs are shown. For all images, total original magnification is × 44/400.