ESR1 and breast cancer: The levels of cdA and cdG, in their R and S diastereoisomeric forms, have been identified in estrogen receptor-alpha (ER-α) MCF-7 and triple-negative MDA-MB-231 breast cancer cells upon exposure to two distinct conditions of radical stress, i.e., 5 Gy of ionizing radiation and 300 μM hydrogen peroxide, followed by an interval period that allows DNA repair [133].