Having the possibility of directing insertion of genetic cargo into desired loci, others used the knock-in potential of CRISPR/Cas9 to safely transfer a tumor suppressor gene, the CCCTC-binding factor (CTCF) into the AAVS1 integration locus of triple-negative breast cancer cells MDA-MB-231 [93]. The gene discussed is CTCF; the disease is triple-negative breast carcinoma.