Editing DNA ligase (section)

==Measurement of activity==
There are at least three different units used to measure the activity of DNA ligase:<ref name="Russell_2001">{{cite book | vauthors = Russell DW, Sambrook J | title = Molecular cloning: a laboratory manual | publisher = Cold Spring Harbor Laboratory | location = Cold Spring Harbor, N.Y | year = 2001 | isbn = 978-0-87969-577-4 | volume = 1 | edition = 3rd | chapter = Chapter 1: Plasmids and Their Usefulness in Molecular Cloning  | pages=1–159 }}</ref>
* '''Weiss unit''' - the amount of ligase that catalyzes the exchange of 1 nmole of <sup>32</sup>P from inorganic [[pyrophosphate]] to ATP in 20 minutes at 37<sup>°</sup>C.  This is the one most commonly used.
* '''Modrich-Lehman unit''' - this is rarely used, and one unit is defined as the amount of enzyme required to convert 100 nmoles of d(A-T)<sub>n</sub> to an exonuclease-III resistant form in 30 minutes under standard conditions.
* Many commercial suppliers of ligases use an arbitrary unit based on the ability of ligase to ligate cohesive ends.  These units are often more subjective than quantitative and lack precision.