Abstract: Compositions and methods for cellular genome engineering that permit simple and efficient targeted knock-in of a CAR and simultaneous knockout of individual genes are described. The compositions and methods are especially applicable to massively parallel engineering, selection, and identification of CAR T cell variants exhibiting a desired phenotype. AAV vectors containing crRNA and CAR expression cassettes and homology arms for targeted genomic integration thereof are provided. Also provided are libraries containing a plurality of AAV vectors and methods of use thereof in screens for identifying desirable CAR T cell variants. Methods of treatment using CAR T cell variants exhibiting improvements in one or more phenotypes are also provided.