Formazan dyes are artificial chromogenic

Chromogenic

Chromogenic refers to color photographic processes in which a traditional silver image is first formed, and then later replaced with a colored dye image.- Description :...

 products of the reduction of tetrazolium salts by dehydrogenase

Dehydrogenase

A dehydrogenase is an enzyme that oxidises a substrate by a reduction reaction that transfers one or more hydrides to an electron acceptor, usually NAD+/NADP+ or a flavin coenzyme such as FAD or FMN.-Examples:...

s and reductases. They have a variety of colors from dark blue to deep red to orange, depending on the original tetrazolium salt used as the substrate for the reaction.

Leading examples of tetrazolium salts, include:

1. INT (2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride), (water insoluble)
2. MTT
   MTT assay
   The MTT assay and the MTS assay are colorimetric assays for measuring the activity of enzymes that reduce MTT or close dyes to formazan dyes, giving a purple color. A main application allows to assess the viability and the proliferation of cells...
   
   (3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyl-2H-tetrazolium bromide) (used in the MTT assay
   MTT assay
   The MTT assay and the MTS assay are colorimetric assays for measuring the activity of enzymes that reduce MTT or close dyes to formazan dyes, giving a purple color. A main application allows to assess the viability and the proliferation of cells...
   
   ), (water insoluble)
3. XTT (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) (water soluble)
4. MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) (used in the MTS assay) (water soluble).
5. TTC
   Tetrazolium chloride
   Triphenyl tetrazolium chloride, TTC, or simply Tetrazolium chloride is a redox indicator commonly used in biochemical experiments especially to indicate cellular respiration....
   
   or Tetrazolium chloride
   Tetrazolium chloride
   Triphenyl tetrazolium chloride, TTC, or simply Tetrazolium chloride is a redox indicator commonly used in biochemical experiments especially to indicate cellular respiration....
   
   (2,3,5-Triphenyl-2H-tetrazolium chloride) (water soluble)

When reduced in a cell, either enzymatically

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 or through direct reaction with NADH or NADPH the classical tetrazolium salt, MTT, turns bright blue and may form an insoluble precipitate. These formazan dyes are commonly used in cell proliferation and toxicity assay

Assay

An assay is a procedure in molecular biology for testing or measuring the activity of a drug or biochemical in an organism or organic sample. A quantitative assay may also measure the amount of a substance in a sample. Bioassays and immunoassays are among the many varieties of specialized...

s like the EpiSkin test since they only stain living cells.