Hannafon BN, et al. - Given the potential relevance of miR-23b and miR-27b in the development of anti-cancer therapeutics, researchers performed genetic depletion of these microRNAs (miRNAs) using CRISPR/Cas9 engineering technology to determine their specific role in breast cancer progression. Developing miR-23b and miR-27b null breast cancer cell lines, they characterized their phenotype in vitro using various phenotypic assays and in vivo using a xenograft mouse model. Reduction in tumor growth in xenograft nude mice fed a standard diet was observed with miR-23b and miR-27b knockout. This supports their oncogenic role in vivo. However, miR-27b depletion, but not miR-23b depletion, compromised fish-oil-induced suppression of xenograft growth when xenograft mice were provided a fish-oil diet. This indicates a context-dependent nature of miR-27b oncogenic activity. Results thereby suggest the primary oncogenic role of miR-23b and miR-27b in MCF7 breast cancer cells and that under certain circumstances, miR-27b may have tumor suppressive activity.