Papain, also known as papaya proteinase I, is a cysteine protease enzyme present in papaya (Carica papaya) and mountain papaya (Vasconcellea cundinamarcensis). Papain belongs to a family of related proteins with a wide variety of activities, including endopeptidases, amoniopeptidases, dipeptidyl peptidases and enzymes with both exo- and endo-peptidase activity.