Men are 3-5 times more likely than women to develop and die from bladder cancer (BCa) but the underlying mechanisms of these gender disparities in BCa are poorly understood. This is highlighted as the major knowledge gaps in the Funding Opportunity Announcement (FOA) “Biology of Bladder Cancer (PAR-19-184)”: 1) “identifying biological mechanisms underlying disparities in sex in bladder cancer incidence”; and 2) “investigating the role of the microbiome in the bladder cancer biology”. The lack of understanding is due in part to the fact that the sex-biasing factors are often confounding co-variants. For example, the sex chromosomes are coupled with the sex hormones. Because of the confounding nature, the impacts of individual sex-biasing factors are exceedingly difficult to quantify. Gut microbiota differs significantly between sexes. Its potential sex- specific role in BCa remains unknown despite a preponderance of evidence that gut microbiota exerts the profound effects on cancer initiation, progression and therapy. To gain insight onto the sex differences in BCa, we will test the hypothesis that the gut microbiota plays a sex-specific role in regulating BCa incidence and mortality. To test this hypothesis, we have designed two specific aims using the innovative genetic and genomic strategies. First, we will determine whether the eradication of microbiota blunts the sex differences in BCa incidence and mortality; And second, we will identify gut microorganisms that are closely associated with BCa incidence and mortality. Upon conclusion, the proposal is expected to significantly advance our knowledge of the gender disparities in cancer and to identify candidate gut microbes that are tightly associated with BCa incidence and mortality.