Background: A high mutation burden a biomarker of response to immune checkpoint therapy in melanoma, non-small cell lung cancer, and colorectal cancer. It is unknown whether mutation burden is predictive of response in other cancer types, and whether it is identifiable using available clinical assays. Methods: Using data for 10,745 tumors from 33 solid cancer types in TCGA, we looked for a mutation burden threshold (iCAM) in each cancer type associated with gene expression signatures of a blocked immune response of robust CD8⁺T cell response and up-regulation of immune checkpoint genes. Results: A unique iCAM threshold was identified in nine cancers: melanoma and lung, colon, endometrial, and gastric adenocarcinoma; serous ovarian, bladder urothelial, cervical, and ER+ HER2− breast cancer. iCAM thresholds applied to published clinical data for patients treated with immune checkpoint therapy showed that iCAM+ patients had significantly better response. iCAM+ tumors can be identified from clinical-grade NGS assays with high accuracy . In a prospective melanoma cohort of patients treated with anti-PD-1 patients with iCAM+ tumors had significantly better objective response rates, progression free survival, and overall survival compared patients to iCAM− tumors. Pattern of somatic mutations were different in iCAM+ and iCAM− tumors, suggesting different mechanism of carcinogenesis in iCAM+ versus iCAM− tumors. Higher fractions of leukocytes were NK (Natural Killer) cells and macrophages were M1 polarized, and a lower fraction of T cells were regulatory T cells in iCAM+ tumors compared to iCAM− tumors. Over 75% of the genes significantly up-regulated in iCAM+ tumors in every cancer types were in the immune response pathway, confirming immune response to be the main differentiator in iCAM+ veruss iCAM− tumors. Conclusions: In nine cancer types, a clinically useful mutation burden threshold (iCAM) associated with gene expression signatures of blocked immune response can identify likely responders to immune checkpoint therapy, and iCAM status is identifiable using currently available clinical NGS assays.