The study was made possible by the embedding of a mechanism of action (MoA) investigation (EME Programme; EME 12/206/52) within the MRC/NIHR-funded UCON clinical trial (EudraCT 2014-003408-65; REC 14/LO/1602) to investigate the efficacy of treatment of HMB with ulipristal acetate (UPA), which is a selective progesterone receptor modulator (SPRM), in a cohort comprising similar sized groups of women with and without fibroids. Here, PGR is linked to leiomyoma.