Proteinase K is a serine protease and the main proteolytic enzyme produced by the fungus Tritirachium album Limber (l). The enzyme has a broad specificity, cleaving peptide bonds C-terminal to a number of amino acids. The enzyme is produced, together with other proteases and an aminopeptidase, during stationary phase when the fungus is grown by submerged culture. The enzyme is so named because the organism can grow on native keratin as sole carbohydrate and nitrogen source owing to the enzyme’s ability to digest keratin. Because of its broad substrate specificity, high activity, and its ability to digest native proteins, proteinase K has found considerable use in procedures where the inactivation and degradation of proteins is required, particularly during the purification of nucleic acids.