Cysteine is likely to be isolated from proteins that are cysteine is likely to be

Cysteine is an amino acid that is likely to be isolated from the intracellular region.

Explanation:

The functional group in cysteine, which is the thiol group is essential for the structural modification of polypeptide after translation. This group does so by the production of the sulfide group after the oxidation of the former group. Thus, cysteine can be isolated from the intracellular region of the cell, since that is where protein post-translational modification occurs. The disulfide bond formed by two cysteine amino acids often leads to cystine.