ESR1 and breast carcinoma: If gut microbiome alterations play a role in breast cancer, the mechanism is still not fully understood, and Baker, Al-Nakkash (84) note that estrogen receptor-positive (ER+) breast cancer has been linked to a hyperactive estrobolome – a collection of genes in commensal bacteria responsible for estrogen metabolism that causes increased intestinal absorption of free estrogen leading to an elevated risk of developing breast cancer.