The American cyclist Tyler Hamilton failed a fluorescent-activated cell sorting test for detecting homologous blood transfusions during the 2004 Olympics. He was allowed to keep his gold medal because the processing of his sample precluded conducting a second, confirmatory test. He appealed a second positive test for homologous transfusion from the 2004 Vuelta a España to the International Court of Arbitration for Sport but his appeal was denied. Hamilton's lawyers proposed Hamilton may be a genetic chimera or have had a 'vanishing twin' to explain the presence of red blood cells from more than one person. While theoretically possible, these explanations were ruled to be of "negligible probability".