A high-profile sports blood-doping case has taken a bizarre twist, with the accused cyclist arguing that testing positive for two distinct types of blood indicates he is a chimera. Last September, Olympian Tyler Hamilton, 34, of Boulder, Colorado, was accused of taking a blood transfusion to boost his performance after a newly developed test showed he had two different types of red blood cells. Hamilton denied the charge, and with the help of geneticist David Housman of the Massachusetts Institute of Technology in Cambridge, he has been arguing that he might be a chimera: an organism with a mix of genetically distinct cells. Human chimeras are not all that rare, Housman told a board arbitrating the case in March. Mothers and fetuses often exchange blood-producing stem cells, and fetuses can also get foreign cells from sharing the womb with a "vanishing twin," he said. But the arbitrators didn't bite, voting 2 to 1 to uphold a 2-year suspension and stating that blood doping was "the only reasonable conclusion." Encouraged by the split decision, however, Hamilton is again appealing, this time to a sport arbitration court in Switzerland. Although other blood samples showed that his minority cell population decreased over several months, Housman says that is consistent with chimerism. Mother-fetus chimerism is unlikely to produce foreign cells at the levels found in Hamilton's blood--about 2%--says geneticist Wendy Robinson of the University of British Columbia in Vancouver, Canada. But Housman predicts that more athletes who are actually chimeras will show up as suspected blood dopers. And, wrote Hamilton this month in his online journal, "If we've accomplished nothing else in this case, we have put a spotlight on the vanishing twin phenomenon." CREDIT: WILLIAM STEIN