Post-mortem interval (PMI) is the time that has elapsed since a person has died. If the time in question is not known, a number of medical and scientific techniques are used to determine it. This also can refer to the stage of decomposition of the body. Post-mortem chemistry, also called necrochemistry or death chemistry, is a subdiscipline of chemistry in which the chemical structures, reactions, processes and parameters of a dead organism is investigated. Post-mortem chemistry plays a significant role in forensic pathology. Biochemical analyses of vitreous humor, cerebrospinal fluid, blood and urine is important in determining the cause of death or in elucidating forensic cases. The vitreous humor is four to five milliliters of colorless gel in the vitreous body of the eye. Because of its location and the inert nature of the vitreous humor, it is resistant to some of the post-mortem changes that occur in the rest of the body. This is what makes it useful in determining the time since death, along with the fact that it is not affected by age, sex, or cause of death. It is also useful as a source of DNA or for diagnosing diseases.