单选题
A mother who is suffering from cancer can pass on the disease to her unborn child in extremely rare cases,【C1】______a new case report published in PNAS this week. According to researchers in Japan and at the Institute for Cancer Research in Sutton, UK, a Japanese mother had been diagnosed with leukemia a few weeks after giving birth,【C2】______tumors were discovered in her daughter's cheek and lung when she was 11 months old. Genetic analysis showed that the baby's cancer cells had the same mutation as the cancer cells of the mother. But the cancer cells contained no DNA whatsoever from the father,【C3】______would be expected if she had inherited the cancer from conception. That suggests the cancer cells made it into the unborn child's body across the placental barrier. The Guardian claimed this to be the first【C4】______case of cells crossing the placental barrier. But this is not the case — microchimerism,【C5】______cells are exchanged between a mother and her unborn child, is thought to be quite common, with some cells thought to pass from fetus to mother in about 50 to 75 per cent of cases and to go the other way about half【C6】______. As the BBC pointed out, the greater【C7】______in cancer transmission from mother to fetus had been how cancer cells that have slipped through the placental barrier could survive in the fetus without being killed by its immune system. The answer, in this case at least, lies in a second mutation of the cancer cells, which led to the【C8】______of the specific features that would have allowed the fetal immune system to detect the cells as foreign. As a result, no attack against the invaders was launched. 【C9】______, according to the researchers there is little reason for concern of "cancer danger". Only 17 probable cases have been reported worldwide and the combined【C10】______of cancer cells both passing the placental barrier and having the right mutation to evade the baby's immune system is extremely low.