Although ALDH1 activity and Aldh1 expression have been identified as a valid marker for BCSLCs [19, 23, 50, 53, 78] and despite ALDHhigh cells having formed tumors sooner than ALDHlow cells in the experiment described above, we wanted to confirm that ALDHhigh cells, in our hands, exhibit a hallmark property of cancer stem cells in vivo, i.e. efficient formation of tumors following xenografting. The gene discussed is ALDH1A1; the disease is cancer.