Human deoxyribonuclease (DNase I) is an important clinical agent currently used in the treatment of cystic fibrosis (CF) patients (1 ). It is inhaled into the airways, where it degrades DNA to lower molecular weight fragments, thus reducing the viscoelasticity of CF sputum and improving lung function (2 ,3 ). In engineering DNase I, our goal was to address several properties that could not only provide more biochemical and pharmacological understanding, but might also result in a more efficacious clinical agent. This goal included engineering DNase I to improve its catalytic activity and to increase its resistance to inhibition by both G-actin and salt.
