Star activity in restriction enzymes is when the restriction enzymes cut recognition sequences that are very similar but not quite identical to the defined sequence of that enzyme. Although all enzymes under certain conditions can exhibit star activity, most of the restriction enzymes that BioLabs supplies, are highly purified and don't exhibit star activity under their normal reaction conditions. If there are organic solvents beside glycerol, like ethanol, DMSO, if there is a large excess of enzymes and the DNA is overloaded with the restriction enzyme, if instead of magnesium, if there is other divalent cations, they can cause the enzyme to have a relaxed specificity, but all these things can contribute to a problem with star activity.
One of the things that NEB does is that we've engineered a number of our restriction enzymes so that they don't undergo star activity and we call those enzymes our HF Enzymes or high fidelity enzymes. Those can be used when your designing experiments if star activity could be a problem under the conditions you're using, then we recommend going to the high fidelity enzymes.
For more information on star activity, you can visit our website which has an extensive amount of information describing this phenomenon and how to avoid it.
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