Gene doping

Gene doping works by injecting genes directly into human cells to blend into an athlete's DNA or by stimulating or blocking natural chemicals. It can increase strength and endurance or enhance muscle growth. The most common technique is using retroviruses, which are injected into the particular tissue of interest. It is the type of virus that enters the nucleus of the cell, makes a break in the DNA, then inserts its own DNA into that break.

Side effects: Unknown, but likely. Since the place where the gene inserts may be random, there's some possibility of the virus breaking apart a normal human gene that has an important role.

Insulin growth factor (IGF-1)

Produced by the liver, IGF-1 stimulates protein synthesis and reduces muscle-cell breakdown, leading to an increase in muscle bulk and reduced body fat. It helps rebuild muscles.

Side effects: Low blood sugar, overgrowth of hands, feet and face; headaches and joint pain; enlargement of the heart.

EPO

Short for erythropoietin, EPO stimulates bone marrow to produce more red blood cells. The rise in cells increases the blood's ability to carry oxygen. The use of EPO is believed to reduce fatigue and improve endurance. Other claims are that it can increase metabolism and the healing in muscles.

Side effects: Flu-like symptoms, headaches, skin rash, high blood pressure and skin irritation. Has been linked to deaths of cyclists who injected it.

Myostatin

The gene for myostatin regulates muscle growth in the body. When removed in mice by researchers at Johns Hopkins in 1997, those mice were two times more muscular than their siblings that contained the same gene intact. Researchers believe the humans can experience the same effect with removal of the gene.

Side effects: Unknown in humans.

Sources: Kyle Serikawa, Australian Sports Drug Agency,The Associated Press, The New Zealand Sunday Star Times,The Seattle Times