Many applications require GroEL in a highly purified form (e.g., see Chapter 10). Since the GroEL sequence does not contain any tryptophan, the detection of tryptophan fluorescence in a preparation of GroEL can be used as a measurement of purity. Following the standard purification procedure as described in Chapter 3, the purified GroEL may show significant tryptophan fluorescence, which has been suggested to be owing to peptides bound to GroEL. An accurate spectroscopic quantification of the GroEL concentration or applications of GroEL in fluorescence measurements require further purification in order to remove traces of tryptophan fluorescence.