All procedures were performed in accordance with the guidelines of the National Institutes of Health animal care committee. One hundred and one thousand HepG2 cells cultured for one week in the presence or absence of varying concentrations of Dox were re-suspended in 200 μl DMEM and Matrigel (BD Bioscience) (1:1) and injected into flanks of NOD/SCID mice (Frederick National Laboratory for Cancer Research). Dox in concentrations of 10 ng/ml and 100 ng/ml was added to drinking water containing 1% sucrose to reduce a bitter taste every two days. Tumor formation was monitored weekly by palpation for 8 weeks. Limiting dilution analysis was performed as described (3 (link)) (http://bioinf.wehi.edu.au/software/elda/index.html).