ESR1 and breast cancer: To further define the respective role of ERα and ERβ in HYAL1 regulation, we generated stable clonal lines using ER-negative human breast cancer MDA-MB-231 cells, which are positive for HYAL1 (Supplementary Figure S2), and in which ERα (231-ERα) or ERβ (231-ERβ) was stably expressed and functional (Supplementary Figure S4).