By examining the oral and gut microbiome of 112 melanoma patients undergoing anti-Programmed Cell Death Protein 1 (PD-1) immunotherapy, significant differences in the diversity and composition of gut microbiomes were observed between responders and nonresponders, and when analyzing the patients’ fecal microbiomes, microbial α-diversity (P < 0.01) and the relative abundance of tumor Clostridiales (P < 0.01) were significantly higher. The gene discussed is PDCD1; the disease is melanoma.