To systematically assess gut microbiome features, the Predicting Response to Immunotherapy for Melanoma With Gut Microbiome and Metabolomics (PRIMM) (NCT03643289) study applied shotgun metagenomic sequencing on stool samples collected from five observational cohorts (n = 165) recruiting ICI-naive patients with advanced cutaneous melanoma who were planned to undergo anti-PD-1 therapy with or without anti-CTLA4 therapy. This evidence concerns the gene CTLA4 and melanoma.