Healthy cells keep deadly ones alive.

• Position: Leukemia

What keeps leukemia cells alive almost forever, able to continue dividing endlessly and aggressively? Research at the Weizmann Institute of Science, Rehovot, Israel, suggests that, in about one-quarter of all leukemias, the cancer cells rely on an internal "balance of terror" to keep going. When one version of a certain gene is mutated, it becomes a cancer-promoting gene--an oncogene. However, new findings show that the second, normal version of the gene, which functions alongside the mutation, is what keeps the cells both cancerous and alive, able to continue forging their destructive pathway in the body.

That gene, RUNX1, is crucial for the development and maintenance of the blood circulatory system. It encodes a transcription factor--a protein that regulates the expression of many other genes. In the blood system, this transcription factor directs the differentiation of certain adult stem cells found in the bone marrow into the various mature blood cells. It only takes a single mutation in the RUNX1 gene in this type of stem cell to send it down the path to becoming a leukemic stem cell.

Acute myeloid leukemia, for example, is characterized by a very specific kind of mutation called a translocation. A bit of genetic material from chromosome 8 makes its way over to the RUNX1 gene on chromosome 21 and inserts itself into the genetic sequence. The result is an oncogene that encodes a fused protein--one that takes on some new functions and loses some old ones. Several other forms of leukemia, including the most prevalent childhood version, acute lymphatic leukemia, begin with a similar...