ATF3 and breast cancer: Specifically, breast-cancer-derived extracellular vesicles transport miR-9-5p to simultaneously target insulin-induced gene 1 (INSIG1)/INSIG2 (endogenous HMGCR suppressors) and activating transcription factor 3 (ATF3), thereby upregulating HMGCR-mediated cholesterol synthesis in coordination and enhancing 25-hydroxycholesterol (25-HC) conversion.