1. A method of reducing cancer cell proliferation comprising introducing into a cancer cell (a) one or more DNA sequences encoding one or more guide RNAs (gRNAs) that are complementary to one or more target sequences in the human NRF2 gene, wherein the human NRF2 gene comprises at least exons 2, 3, 4 and 5 and (b) a nucleic acid sequence encoding a Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)-associated endonuclease, whereby the one or more gRNAs hybridize to the human NRF2 gene and the CRISPR-associated endonuclease cleaves the human NRF2 gene, and wherein human NRF2 gene expression or activity is reduced in the cancer cell relative to a cancer cell in which the one or more DNA sequences encoding the one or more gRNAs and the nucleic acid sequence encoding the CRISPR-associated endonuclease are not introduced thereby reducing proliferation of the cancer cell by at least 5% relative to a cancer cell that is not treated with the one or more DNA sequences of (a) and the nucleic acid sequence of (b); wherein the cancer cell is a non-small-cell lung cancer cell, a head and neck cancer cell, or an esophageal squamous cancer cell.