RARA and breast cancer: Conversely, as shown here in a mammary epithelial cell context, a dysfunctional RARA transcriptional arm can determine: a) the physiological RA cancer-promoting action leading to aberrant mammary morphogenesis, which is a feature of breast cancer initiation, b) differential cancer-promoting actions of physiological (endogenous) RA during cancer progression, and even c) paradoxical anticancer and cancer-promoting actions of supraphysiological (exogenous) RA used for cancer treatment.