The six caught via the "biological passport" method were flagged as suspicious in 2009 and were found to have persistently used banned substances over a long period, the IAAF said.

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Five years ago the government lifted the anonymity granted to donors, so that now any offspring conceived by the donor method, are able to trace their biological parents when they get to the age of 18.

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If there is a biological attack on U.S. soil, Cantor says, the method could be used to screen hundreds of thousands of patients to see if they are infected.

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