Inclusion Criteria:  Men or women ≥18 years of age Screened negative for COVID-19 symptoms at time of consent, as per institutional policy and as applicable for the duration of the COVID-19 pandemic Diagnosed with stages I-IV primary NSCLC, MM, TNBC or RCC Plan to be treated at a partner cancer site with a checkpoint inhibitor (anti-PD-1, anti-PD-L1, or anti-CTLA-4) as a single agent or in combination with another checkpoint inhibitor or other treatment agent or modality (e.g., targeted therapy, chemotherapy, surgery, radiation, etc.) in accordance with FDA-labeled use of the agent Able to provide informed consent and answer study questionnaires in either English or Spanish Able to provide stool specimens for research purposes  Exclusion Criteria:  Mental incapacity Incarcerated individuals Pregnancy (by self-report of pregnancy status) Experiencing active brain metastasis/metastases Treatment with checkpoint inhibitor in off-label capacity or through a clinical/interventional trial Active participation in an immuno-oncology clinical/interventional trial or pharma-sponsored observational study