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. Here, ESR1 is linked to breast cancer.