Cystine is a Non-Essential Disulfide Amino Acid formed within the body by linking two Cysteine molecules by their Sulfur atoms - i.e. Cystine is the oxidized form of Cysteine. Cysteine and Cystine readily convert to one another and have almost identical functions within the body.
Methylsulfonylmethane is a type of Sulfuric Compound that is naturally present in the body (i.e. it is an endogenous form of Sulfur). MSM is similar in chemical structure to (but claimed to be safer than) Dimethyl Sulfoxide (DMSO) - MSM is the active therapeutic constituent of DMSO and is also an oxidative metabolite of DMSO. It is odorless, tasteless and stable. Its physical format is a white crystalline powder.