"The 'help' has evolved over the years but the fact remains that our sport is damn hard, the Tour was invented as a stunt, and very tough (riders) have competed for a century and all looked for advantages, from hopping on trains 100 years ago to EPO now, " he said.

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Testing for EPO directly is difficult because the synthetic version is almost indistinguishable from its natural form - a version of the hormone was originally created to treat severe anaemia.

BBC: Starting blocks

And while EPO's effect is long-lasting, the hormone itself rapidly disappears from the body.

BBC: Starting blocks

Secreted proteins, which include growth hormone, insulin and EPO (erythropoietin), are used by the body as molecular messengers to pass signals from one part of the body to another.

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