Abstract: Provided are compositions and methods for the production of modified endonucleases such as CRISPR/Cas9 system with reduced off-target activity. Methods of editing of polynucleotides using the modified endonucleases in vitro and in vivo are also disclosed. In one aspect, the disclosure provides a modified endonuclease, comprising an endonuclease and one or more mixed charge moieties covalently linked to the endonuclease, wherein each mixed charge moiety comprises about 10 to about 400 positively charged moieties and about 10 to about 400 negatively charged moieties, and wherein the ratio of the number of positively charged moieties to the number of negatively charged moieties is from about 1 :0.5 to about 1 :2.