O-GlcNAcylation-inducing treatments with O-GlcNAcylation PUGNAc and glucosamine inhibited ERα expression, which protected MCF-7 cells from 4-OH-tamoxifen-induced apoptosis by stimulating the PI3K/AKT signaling pathway and conferred tamoxifen resistance in breast cancer [205]. The gene discussed is ESR1; the disease is breast cancer.