expression on tumor cell shedding into the bloodstream. (A) Numbers of circulating tumor cells per milliliter of blood isolated from five mice bearing control MCF7 cells and five mice with MCF7 3C3 tumors. Tumor cells were isolated from mice with equal tumor volume (1 cm3 per mouse), and non-tumor-bearing mice were used as a control. Cells from each group were sorted into one well of a 96-well plate, and (B) the numbers of cells isolated from the circulation with clonogenic potential are shown. (*P < 0.005, compared with MCF7 control cells, by one-way ANOVA followed by the Dunnett two-sided multiple-comparison test).