Star activity is a relaxation or alteration of the specificity of restriction enzyme

Restriction enzyme

A Restriction Enzyme is an enzyme that cuts double-stranded DNA at specific recognition nucleotide sequences known as restriction sites. Such enzymes, found in bacteria and archaea, are thought to have evolved to provide a defense mechanism against invading viruses...

 mediated cleavage of DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 that can occur under reaction conditions that differ significantly from those optimum for the enzyme. The result is typically cleavage at non-canonical

Canonical

Canonical is an adjective derived from canon. Canon comes from the greek word κανών kanon, "rule" or "measuring stick" , and is used in various meanings....

 recognition site, or sometimes complete loss of specificity.

Differences which can lead to star activity include low ionic strength

Ionic strength

The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts...

, high pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

, and high (> 5% v/v) glycerol

Glycerol

Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

 concentrations. The latter condition is of particular practical interest, since commercial restriction enzymes are usually supplied in a buffer

Buffer solution

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...

 containing a substantial amount of glycerol (50% v/v is typical), meaning insufficient dilution of the enzyme solution can cause star activity; this problem most often arises during double or multiple digests.
Star activity can happen because of presence of Mg²⁺, as is seen in HindIII, for example.

External links

• Star Activity - New England Biolabs
  New England Biolabs
  New England Biolabs produces and supplies reagents for the life science industry. NEB offers 230 recombinant and 30 native restriction enzymes for genomic research, as well as nicking enzymes and DNA methylases. It offers products in the areas related to proteomics, DNA Sequencing, and drug...
  
  
• Star Activity (Relaxation of Specificity) - Fermentas
• Star activity of restriction enzymes - a detailed list from TaKaRa

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2396408
The Fidelity Index provides a systematic quantitation of star activity of DNA restriction endonucleases