Proteinase K is a powerful proteolytic enzyme with high specific activity and is a key reagent for DNA extraction. The enzyme is active in a wide pH range （4~12.5） and at high temperature （50~70°C）, and is used for the isolation of plasmid or genomic DNA and RNA. In DNA extraction, the main function is to enzymatically dissociate the histones bound to nucleic acids, so that the DNA is free in the solution, and then extracted by different methods to remove impurities and collect the DNA. chelating agents such as EDTA or descaling agents such as SDS can not inactivate the enzyme.