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You are an expert summarizer. Condense information accurately and concisely, highlighting key points and main ideas from the provided text, tailored to the requested length or style.<|im_end|>
<|im_start|>user
Summarize the following news article:
(CNN) -- Ricin, a poison thought to have been found in a Las Vegas, Nevada, hotel room Thursday, can be made from the waste left after processing castor beans, according to the Centers for Disease Control and Prevention. Ricin is made from castor beans. The toxin comes in the form of a mist or pellet and can be dissolved in water or weak acid, according to the CDC. It works by getting inside the cells of the body and preventing them from making the proteins they need. As little as 500 micrograms -- an amount the size of the head of a pin -- can kill an adult. Here are some ricin cases: . • September 1978, London, England: Bulgarian dissident Georgi Markov is killed by a ricin-filled poison dart fired from an umbrella. Markov, a communist defector working for the BBC World Service, was waiting at a bus stop when he was killed. • 2002, Iraq: A primitive testing facility run by members of Ansar al Islam, a Kurdish Sunni Islamist group, is discovered. Authorities say ricin had been tested on barnyard animals there. • January 2003, United Kingdom: Scotland Yard arrests seven terror suspects from Algeria after traces of ricin are discovered at their homes. A U.S. official later connects the men to Abu Musab al-Zarqawi, who led al Qaeda in Iraq until his death in 2006. Four of the men were charged with terrorism offenses under Britain's Terrorist Act 2000 and with "being concerned in the development or production of chemical weapons" under the Chemical Weapons Act of 1996. • March 2003, France: Small bottles containing traces of ricin are found in a Paris train station, according to French police. • October 2003, United States: Ricin is found in a sealed envelope in a postal handling facility in Greenville, South Carolina.  Watch CNN's Dr. Sanjay Gupta explain the potency of ricin » . • November 2003, United States: The Secret Service intercepts a letter addressed to the White House that contains a vial of ricin. The letter, signed by "Fallen Angel," complained about trucking regulations, and was nearly identical to one discovered October 15 in South Carolina. • February 2004, United States: Ricin is found in the mailroom of the Dirksen Senate Office building in Washington D.C. The mailroom handled correspondence addressed to then-Senate Majority Leader Bill Frist and others. Frist said no one became sick. • January 2005, United States: An Ocala, Florida, man with no known ties to terrorists or extremists is arrested by the FBI after agents found ricin in the home he shares with his mother. Steven Michael Ekberg pleaded guilty in federal court to possession of a biological weapon. E-mail to a friend .<|im_end|>
<|im_start|>assistant
Ricin found in four U.S. cases since 2003 .
Bulgarian dissident killed by dart containing ricin in 1978 .
500 micrograms of ricin, size of a  pinhead, can kill an adult .<|im_end|>
