The cancer stem cell hypothesis continues to show promise as a way to strike at the root of many different forms of cancer: "Cancer stem cells are defined by three abilities: differentiation, self-renewal and their ability to seed a tumor. These stem cells resist chemotherapy and many researchers posit their role in relapse. A [new study] shows that melanoma cells with these abilities are marked by the enzyme ALDH, and imagines new therapies to target high-ALDH cells, potentially weeding the body of these most dangerous cancer creators. ... We've seen ALDH as a stem cell marker in other cancer types, but not in melanoma, and until now its function has been largely unknown. ... [Researchers] transplanted ALDH+ and ALDH- melanoma cells into animal models, showing the ALDH+ cells were much more powerfully tumorigenic. In the same ALDH+ cells, the group then silenced the gene that creates this protein, finding that with ALDH knocked down, melanoma cells died in cultures and lost their ability to form tumors in animal models. In cell cultures, silencing this ALDH gene also sensitized melanoma cells to existing chemotherapies. When the group explored human tumor samples, they found distinct subpopulations of these ALDH+ cells, which made up about 0.1-0.2 percent of patients' primary tumors. In samples of metastatic melanoma - the most aggressive form of the disease - the percentage of ALDH+ cells was greater, even over 10 percent in some tumors, further implying the powerful danger of these cells."