Sarin, the deadly nerve agent the US says the Syrian regime used on a "small scale", was developed by Nazi scientists in the 1930s.
Originally developed as a pesticide, sarin was used to deadly effect in the 1988 raid on the Kurdish village of Halabja in northern Iraq. A Japanese cult also used sarin in two attacks in the 1990s.
The gas works by being inhaled or absorbed through the skin and kills by crippling the nervous system.
Symptoms include nausea and violent headaches, blurred or tunnel vision, drooling, muscular convulsions, respiratory arrest, loss of consciousness and then death, according to the US Centers for Disease Control and Prevention.
In high doses, sarin paralyses the muscles around the lungs and prevents chemicals from "switching off" the body's secretions, so victims suffocate or drown as their lungs fill with mucus and saliva.
Even a tiny dose of sarin -- which, like other nerve gases such as soman, tabun and VX, is odourless, colourless and tasteless -- can be deadly if it enters the respiratory system, or if a drop comes into contact with the skin.
Even if it does not kill, sarin's effects can be permanent, inflicting lasting damage to the victim's lungs, eyes and central nervous system.
Being heavier than air, sarin can linger in an area for up to six hours, depending on weather conditions.
The most notorious sarin attack occurred in March 1988 in Halabja when as many as 5,000 Kurds were killed and 65,000 injured when the Iraqi military used a combination of chemical agents including sarin, mustard gas and possibly VX.
Sarin killed 13 people and injured 6,000 others when the Aum Supreme Truth cult released it in the Tokyo subway in March 1995. The cult had used the nerve agent in a deadly attack the year before in the Japanese city of Matsumoto.
Syria's stockpiles contain hundreds of tonnes of various chemical agents, according to Leonard Spector of the Monterey Institute of International Studies.
"Their arsenal of chemical agents is rather robust, " said Olivier Lepick of the Paris-based Foundation for Strategic Research.
In addition to blister agents known as vesicants such as mustard gas (yperite), Damascus is believed to possess sarin and possibly VX, another nerve agent 10 times more powerful than sarin.
The Syrian regime is also said to possess delivery systems to use its chemical weapons, such as Scud missiles, artillery shells and aerial bombs.