Diisopropyl fluorophosphate (DFP) - Sarin's little brother

Organophosphates are a class of chemical compounds that possess a phosphate atom bound to a couple of oxygen atoms. They include among their ranks such essential-for-life biochemicals as nucleic acids and ATP, as well as life-ending insecticides, herbicides, and nerve agents.

Nerve agents kill you by inhibiting an enzyme called acetylcholinesterase, which hangs out wherever nerves from your brain/spinal cord connect with your muscles in order to tell them what to do. When the nervous system wishes a muscle to bend to its will (lol), a neurotransmitter (chemical facilitator of communication between nerve and muscle) called acetylcholine is released from nerves and binds to sites on muscle to cause them to contract. Normally, acetylcholinesterase then breaks down the acetylcholine so that the muscle can relax. Organophosphates bind to a special site on acetylcholinesterase and prevent it from breaking down acetylcholine, resulting in prolonged muscle contraction (i.e. being paralyzed). Since you need muscles to breathe, organophosphates are somewhat problematic, causing asphyxiation and death death death.

Diisopropyl fluorophosphate (DFP) is an organosphate nerve gas; an insecticide gone wrong, if you will. It was originally developed by a British dude named Bernard Charles Saunders, who was trying to break into the exciting world of chemical warfare products (aren't we all). In a major setback for Mr. Saunders, DFP was found to be less toxic than similar compounds like tabun or sarin. However, he stuck with his little toxic friend, and eventually figured out that it could be mixed with mustard gas to produce a mixture with a sufficiently low enough melting point to be used in cold weather, which was apparently a big deal at the time. Mustard gas: Now able to blister you to hell all year round.

Since DFP resembles much more toxic chemical agents (taurin, sarin, soman, cyclosarin, etc.), it has been used by military forces as a substitute for such agents in training drills and top-secret experiments and stuff. DFP is actually a structural analog of sarin, meaning that it contains the same atoms but they are arranged differently.

DFP has been used by eye doctors and vets to produce miosis (constriction of the pupil of the eye), which is beneficial to the treatment of chronic glaucoma and some other stuff. Neuroscience researchers have been known to play around with it since it inhibits acetylcholinesterase and so can be used to induce delayed peripheral neuropathy, permitting this condition to be studied.