This page describes some of the different isotopic labelling strategies that are commonly used in Protein NMR.
In most cases (except in cell-free labelling), the protein is expressed by bacteria. The isotopic labels are introduced by feeding the bacteria specific nutrients. In most cases the basis will be so-called minimal medium which contains all the salts and trace elements needed by the bacteria but contains no carbon or nitrogen sources. These elements can then be introduced using a variety of different isotopically labelled carbon and nitrogen sources.