Thus, CRISPR-Cas9-mediated introduction of mutations in the genomically encoded ESR1 gene would facilitate direct comparison of isogenic wild-type and mutant breast cancer cells towards developing a better understanding of the consequences of these mutations on response to endocrine therapies, and importantly, would enable evaluation of therapeutic approaches to target breast cancers featuring ESR1 mutations. The gene discussed is ESR1; the disease is breast carcinoma.