Provided are medical uses and methods for targeting cancer stem cells employing ProTide compounds, particularly in the prevention or treatment of cancer. The ProTide may be other than one selected from the group consisting of : NUC-1031; a ProTide derived from cordycepin; and a ProTide derived from 8-chloroadenosine. The medical uses and methods for targeting cancer stem cells are particularly useful in the treatment of relapsed or refractory cancer in human patients. The invention also provides methods of selecting patients who will benefit from prevention or treatment of cancer through the medical uses or methods of treatment of the invention.