Abstract: Described are methods comprises transducing a mammalian cell with one or more virus vectors. Each vector comprises a nucleic acid sequence encoding a Cpf1 (also known as Cas12a) protein and an optional selectable marker in operative association with an RNA pol II promoter which controls expression thereof; and a CRISPR RNA (crRNA) array comprising at least two spacers in operative association with an RNA pol III promoter. Each spacer encodes an RNA guide which hybridizes to a unique sequence located 3? from a T-rich protospacer-adjacent motif (PAM) in a genomic region of interest. The method further comprises culturing the transduced cells, thereby providing a plurality of cultured cell cultures, each cell culture comprising said deletion. Additionally, described are compositions used in methods as well as libraries generated by the methods. Such compositions comprise libraries of transduced cell cultures, viral vectors, nucleic acid sequences, CRISPR RNA spacers, and RNA guides, as described herein.