VX is a deadly chemical agent classified as a weapon of mass destruction by the United Nations. Only a few countries are known to produce it.
How powerful is VX?
VX is the most potent and rapidly acting of all known chemical warfare agents. A single drop is enough to kill within minutes.
"The "V" in VX stands for "venom," a tribute to this compound class having high potency and a characteristic ability to penetrate the skin," according to the US-based National Academy of Sciences.
VX is outlawed under the Chemical Weapons Convention (CWC).
Its only known use is as a chemical warfare agent.
How was it used in the murder?
Kim Jong-nam, the half-brother of North Korea's leader, was assaulted by two women who, according to Malaysian police, rubbed the chemical on his face at Kuala Lumpur International Airport.
Police arrested the two women and a North Korean man last week. They are also seeking seven other North Koreans in connection with the case, including a diplomat at the embassy in Kuala Lumpur.
South Korean and US officials have said they believe North Korean agents assassinated Kim Jong-nam, who had been living under China's protection.
North Korea has denied the allegations.
How did a banned chemical weapon find its way into an international airport?
Since it is tasteless and odourless, it is unlikely that airport sensors or security could have detected small amounts.
Malaysian police said they are investigating whether the illegal agent was brought into the country or made there.
South Korean analysts said Sarin and VX are the focus of a North Korean chemical weapons program.
When has VX been used in the past?
VX and other nerve agents were believed to have been used in chemical warfare during the Iran-Iraq War in the 1980s.
In 2015, traces of VX and Sarin, another nerve agent were found at a military research site in Syria.
A mix of Sarin and VX was used by members of a Japanese doomsday cult in their deadly 1995 attack on a Tokyo subway.