Inducible tumor formation of TonB210.1 cells in nude mice. 10⁷ TonB210.1 cells were injected subcutaneously into the flanks of nude mice given water without (left) or with (middle) 400 μg/mL doxycycline. Tumor regression was observed following removal of doxycycline from the drinking water (right). Sites of TonB210.1 injections resulting in tumors are indicated by arrows.