I've been reading The Sports Gene, and came across this remarkable information which I hadn't been aware of:
Keep in mind that while this gene would not affect response to the isotope test, because the latter is expensive and time-consuming , it generally is not used unless the athlete first fails the T/E screening test.About ten percent of people with European ancestry have two copies of a gene variant that allows them to dope with impunity…carriers of two copies of a particular version of the UGT2B17 gene pass the [T/E testosterone] test no matter what. The gene is involved in testosterone excretion and one version of it causes the T/E ratio to remain normal no matter how much testosterone one injects. So ten percent of European athletes can cheat [with impunity on the T/E test]…[This] gene is more rule than exception in other parts of the world, like East Asia. Two-thirds of Koreans have [it].