Mitochondria’s role in cancer has been hypothesized since the 1930s, when Otto Warburg, a Nobel Prize–winning German physician, discovered that mitochondria in cancer cells do a poor job of producing energy. And although the past 70 years have seen often halting progress in research on the subject, it’s now clear there is at least a coincidental connection between mitochondria and cancer. “All cancer cells examined so far have mutations in mitochondrial DNA,” says Keshav Singh, a mitochondrial geneticist at the Roswell Park Cancer Institute in Buffalo and founder of the Mitochondria Research Society. “And most cancer cells have defective mitochondria.”

But as with diabetes, the question is whether mitochondrial mutations or defects cause cancer or are merely correlated with it. Singh believes that defective mitochondria harm other parts of the cell, particularly the nucleus. “When mitochondria don’t work properly, they cause genetic instability in the nuclear genome,” Singh says, “and instability is a major cause of cancer.” The result is that mutations occur more frequently than normal, increasing the likelihood that genes involved in cell division or cell death will fail to work. (Uncontrolled cell division leads to cancer; programmed cell death prevents it.)

That instability may stem from mitochondria that produce an excess of free radicals, which can damage nuclear DNA. Or, Singh says, deficient mitochondria may trigger a process called error-prone repair—instead of fixing damaged DNA, it inserts incorrect nucleotides into the genetic code, increasing the likelihood that a cell will mutate and become cancerous.

No one yet knows which comes first—a malignant tumor or mitochondrial mutations—but the association itself may aid cancer detection. The existence of mutations could signal a cancer’s presence, and with recent breakthroughs in scanning bodily fluids for mitochondrial mutations, it may be possible to screen for early signs of disease. (Because there is a lot of mitochondrial DNA in cancer cells, mutations are easy to spot, and they appear early in cancer development.)