We observed a highly significant difference in the species richness of the gut microbiome between placebo and letrozole-treated mice over the course of the study. Specifically, there was a strong positive correlation between bacterial species richness and time in placebo-treatedmice and no correlation in letrozole-treated mice (Fig 2A and 2B). A similar, even more robust, pattern was observed using Faith’s PD estimate (Fig 2C and 2D), indicating that the changes in species richness was spread across phylogenetically diverse groups of bacteria. Direct compari- son of placebo and letrozole treated mice demonstrated a significant decrease in phylogenetic diversity after 5 weeks of letrozole treatment while the decrease in species richness was not sta- tistically significant, likely due to the relatively small number of mice per group. Generally speaking, higher overall biodiversity is known to improve ecosystem function and productivity in macrobiotic systems. More recently, the species richness of the gut microbial ecosystem has also been correlated with human health.