A study by Martinez-Outschoorn et al. [64] on human cell lines demonstrated that loss of function in BRCA1 mutation led to the production of hydrogen peroxide and oxidative stress in epithelial breast cancer cells and in the stromal fibroblast microenvironment, and resulted in an increased expression of monocarboxylate transporter 4 (MCT4) to shuttle L-lactate out of cells. The gene discussed is BRCA1; the disease is breast cancer.