proteinase K is supplied as a lyophilized powder. It is a serine proteinase that has broad substrate spectrum. Proteinase K was originally isolated from a fungus able to grow on keratin and the enzyme can digest native keratin (hair), then the enzyme was named "Proteinase K". Proteinase K is frequently used in molecular biology applications to digest unwanted proteins, such as protein contents in mouse tail, nucleases from DNA or RNA preparations from cultured cells, and plants.