Cysteine

Cysteine (abbreviated as Cys or C) is an α-amino acid with the chemical formula HO2CCH(NH2)CH2SH. It is a semi-essential amino acid, which means that it can be biosynthesized in humans.[4] It is encoded by the codons UGU and UGC. The thiol side chain in cysteine often participates in enzymatic reactions, serving as a nucleophile. The thiol is susceptible to oxidization to give the disulfide derivative cystine, which serves an important structural role in many proteins. When used as a food additive, it has the E number E920.

Applications
Cysteine, mainly the L-enantiomer, is a precursor in the food, pharmaceutical, and personal-care industries. One of the largest applications is the production of flavors. For example, the reaction of cysteine with sugars in a Maillard reaction yields meat flavors.L-Cysteine is also used as a processing aid for baking.
In the field of personal care, cysteine is used for permanent wave applications, predominantly in Asia. Again, the cysteine is used for breaking up the disulfide bonds in the hair's keratin.
Cysteine is a very popular target for site-directed labeling experiments to investigate biomolecular structure and dynamics. Maleimides will selectively attach to cysteine using a covalent Michael addition. Site-directed spin labeling for EPR or paramagnetic relaxation enhanced NMR also uses cysteine extensively.
In a 1994 report released by five top cigarette companies, cysteine is one of the 599 additives to cigarettes. Like most cigarette additives, however, its use or purpose is unknown.Its inclusion in cigarettes could offer two benefits: acting as an expectorant, since smoking increases mucus production in the lungs; or increasing the beneficial antioxidant glutathione (which is diminished in smokers).
Reducing toxic effects of alcohol
Cysteine has been proposed as a preventative or antidote for some of the negative effects of alcohol, including liver damage and hangover. It counteracts the poisonous effects of acetaldehyde. Cysteine supports the next step in metabolism, which turns acetaldehyde into the relatively harmless acetic acid. In a rat study, test animals received an LD50 dose of acetaldehyde. Those that received cysteine had an 80% survival rate; when both cysteine and thiamine were administered, all animals survived.No direct evidence indicates its effectiveness in humans who consume alcohol at low levels.
N-Acetylcysteine
N-Acetyl-L-cysteine is a derivative of cysteine wherein an acetyl group is attached to the nitrogen atom. This compound is sold as a dietary supplement, and used as an antidote in cases of acetaminophen overdose,
and obsessive compulsive disorders such as trichotillomania.
Sheep
Cysteine is required by sheep to produce wool: It is an essential amino acid that must be taken in from their feed. As a consequence, during drought conditions, sheep produce less wool; however, transgenic sheep that can make their own cysteine have been developed.
Dietary Restrictions
The presence of L-Cysteine is often a point of contention for people following dietary restrictions such as Kosher, Halal, Vegan or Vegetarian as it may be sourced from various human or animal sources. 
As a result an increasing amount of L-Cysteine is produced via a microbial or other synthetic processes