Blood doping? Or chimerism?

When the news hit that American cyclist Tyler Hamilton had been suspended for two years because of evidence that he had used blood transfusions to boost performance, I was pretty shocked. I didn't want to believe that a guy who rode nearly the entire Tour de France with a broken collar bone could be cheating. Or that a guy so devoted to his dog could be a bad guy.

So today's story in the Times' Science section is both fascinating and encouraging for the more hopeful/naive cycling fans among us.

Last month, when the champion American cyclist Tyler Hamilton was accused of blood doping, or transfusing himself with another person's blood to increase his oxygen-carrying red cells, he offered a surprising defense: the small amount of different blood found mixed in with his own must have come from a "vanishing twin."

In other words, his scientific expert argued, Mr. Hamilton had a twin that died in utero but, before dying, contributed some blood cells to him during fetal life. And those cells remained in his body, producing blood that matched the dead twin and not Mr. Hamilton. Or perhaps it was his mother's blood that got mixed in during fetal life.

An arbitration panel did not believe those hypotheses and said there was a "negligible probability" that Mr. Hamilton was anything but guilty.

The test, they concluded in a 2-to-1 decision, shows a blood transfusion and that meant that Mr. Hamilton was suspended from racing for two years, the first and only person convicted for that offense. At age 34, near the end of his career, it could mean his championship days are over.

[...]

Whether Mr. Hamilton is guilty or innocent, his defense does refer to a real phenomenon. Researchers who have no involvement in Mr. Hamilton's case say it actually is possible for someone to have two types of blood in his body, without doping. They emphasize that they do not know whether this is the case with Mr. Hamilton.

One route to this odd state, called chimerism, is the vanishing twin. Dr. Helain Landy of Georgetown University, who has no involvement in the Hamilton case, has found that 20 to 30 percent of pregnancies that start out as twins end up as single babies, with one twin being absorbed by the mother during the first trimester.

Others researchers have found that in some cases, before the twin is absorbed, some of its cells enter the body of the other fetus and remain there for life. The cells can include bone marrow stem cells, the progenitors of blood cells.

Another route to chimerism is through the cells that routinely pass from a mother to fetus and remain there for life.

But I'm afraid that in the competitive world of pro cycling, where every cyclist is suspected and the regulatory bodies struggle to stay abreast of the latest doping science, you're guilty until proven innocent.

And, he said, reports of people with small amounts of foreign cells do not signify that an athlete with a second population of blood cells had someone else's blood stem cells in his bone marrow. Moreover, he said, Mr. Hamilton tested negative a few months after his positive test last fall. That is consistent with an athlete who had transfusions, was caught, and then stopped.

In addition, Dr. Brown said, another rider on Mr. Hamilton's team, Santiago Perez, also tested positive. (He did not show up for his hearing and was pronounced guilty in absentia.)

"It seems inconceivable to me that there would be two people who were rare chimeras on the same cycling team," Dr. Brown wrote by e-mail.

Dr. Olivier Rabin, the scientific director of the World Anti-Doping Agency, said the onus was on Mr. Hamilton to prove he was innocent. "It is up to the defending party to prove it is anything other than blood doping," he said.

With the help of a molecular biology professor at MIT, Hamilton may yet make his case. But the wheels of "justice" in international sports turn slowly and usually against the athlete.