Mayo Clinic researchers reported real-world gut microbiome differences across colorectal and breast cancers in a cohort spanning multiple cancer groups and treatment experiences. In the Mayo Clinic Cancer Microbiome study (MCCM), the team analyzed stool samples from 1,364 patients before therapy and found that microbiome signatures varied by cancer type and by age at diagnosis. The study also linked distinct bacterial patterns to treatment adverse events, strengthening the case that microbiome profiling could become part of risk stratification for oncology tolerability. The authors emphasized that moving from association to causality will require additional work to prioritize microbes and pathways likely to drive outcomes. For precision oncology programs, the key takeaway is that baseline microbiome mapping is now being treated as an actionable variable—rather than a passive biomarker—when building patient-level expectations for therapy.