This is a single-center, investigator-initiated, non-interventional study evaluating the role of the intestinal microbiome and autoimmune panels as a predictor for developing ≥ Grade 2 CTCAE v5.0 immune-related adverse event (irAE) and/or requiring systemic immunosuppression for irAEs in advanced solid tumor patients receiving immunooncology (IO) combinations at the Princess Margaret Cancer Centre. This is a minimal risk study involving the analysis of patient samples and does not involve therapeutic intervention. The study will involve a prospective cohort of up to 120 patients and it is anticipated that patient accrual will be completed within 18 months. Patients will receive IO combination as per their specific protocols from their other clinical trial or per their standard of care and samples will be collected at multiple time-points. No additional visits to the hospital will be needed for this study as safety assessments are already captured for all patients based on their participation in a clinical trial or per their standard of care.