Abstract: The present disclosure provides methods for preparing a population of genetically-modified immune cells. The methods include contacting a population of immune cells with lipid nanoparticles in the presence of an apolipoprotein. The lipid nanoparticles include mRNA encoding an engineered nuclease having specificity for a recognition sequence in the genome of the immune cells. The mRNA is delivered into the immune cells and the engineered nuclease is expressed, generating a cleavage site at the recognition sequence. Further provided are populations of genetic ally-modified immune cells produced according to the disclosed methods, pharmaceutical compositions containing such cells, and methods of treating diseases with the genetically-modified immune cells.