A mother’s cancer cells can be passed along to an unborn child, a team of British researchers has shown. This goes against previously held beliefs that the child’s immune system should block a mother’s cancer, even though rare cases exist in which a mother and child appear to share the same cancer; indeed, there are records of 17 cases of a mother and baby apparently sharing the same cancer, usually melanoma or leukemia. The question of whether a mother can pass along cancer to a child has puzzled scientists for over 100 years, the BBC reports. In the latest study, published in the Proceedings of the National Academy of Sciences, researchers used an advanced genetic fingerprinting technique to show that the leukemia cells present in the baby came from the mother. They also looked at how the cancer cells could have neutralized the baby’s immune system, and found the cancer cells lacked some DNA that plays an important role in helping the immune system recognize cells as foreign. The immune system, then, wasn’t mobilized to attack them. “We are pleased to have resolved this longstanding puzzle. But we stress that such mother-to-offspring transfer of cancer is exceedingly rare and the chances of any pregnant woman with cancer passing it on to her child are remote,” said lead author Professor Mel Greaves from the Institute of Cancer Research.