Proteinase K is a serine protease isolated from a fungus that is able to digest keratin. It has broad substrate specificity and can degrade many native proteins even in the presence of detergents. Proteinase K is commonly used in molecular biology to digest unwanted proteins from nucleic acid preparations. It works best at pH 7.5-8.0 and 37°C, and is typically used at concentrations of 50-200 μg/ml for 30 minutes to 18 hours. Proteinase K is stable and active over a broad range of conditions.