ESR1 and breast cancer: In the present study, we showed that treatment of estrogen receptor/progesterone receptor-positive MCF-7 and triple-negative MDA-MB-231 human breast cancer cells with the GnRH-II antagonist [Ac-D2Nal1, D-4Cpa2, D-3Pal3,6, Leu8, D-Ala10]GnRH-II resulted in apoptotic cell death via activation of stress-activated MAPK p38 and loss of mitochondrial membrane potential.