Every single cell clones from cancer cell lines growing tumors in vivo may not invalidate the cancer stem cell concept.

We present the result of our research on the tumorigenic ability of single cell clones isolated from an aggressive murine breast cancer cell line in a matched allografting mouse model. Tumor formation is basically dependent on the cell numbers injected per location. We argue that in vivo tumor formation from single cell clones, isolated in vivo from cancer cell lines, may not provide conclusive evidence to disprove the cancer stem cell (CSC) theory without additional data.