The article discusses the evolving landscape of biomarkers for checkpoint inhibitor immunotherapy, focusing on factors influencing response, resistance, and adverse effects. Checkpoint inhibitors targeting CTLA4 or PD1 pathways have shown success in cancer treatment, but only a subset of patients benefit. Recent research highlights the role of tumour genomics, host germline genetics, PD1 ligand 1 (PDL1) levels, tumour microenvironment, and gut microbiome in determining ICI response. Key factors include tumour mutation burden (TMB), neoantigens, and immune checkpoint molecules like PDL1. TMB is associated with ICI response, but other factors such as tumour heterogeneity, immune cell infiltration, and genetic mutations also play roles. PDL1 expression is a biomarker for ICI response, but its predictive value is limited. Tumour-infiltrating lymphocytes (TILs) and immune-inflamed, immune-excluded, and immune-desert phenotypes are also important in determining ICI outcomes. Understanding these factors is crucial for optimizing immunotherapy and improving patient outcomes.The article discusses the evolving landscape of biomarkers for checkpoint inhibitor immunotherapy, focusing on factors influencing response, resistance, and adverse effects. Checkpoint inhibitors targeting CTLA4 or PD1 pathways have shown success in cancer treatment, but only a subset of patients benefit. Recent research highlights the role of tumour genomics, host germline genetics, PD1 ligand 1 (PDL1) levels, tumour microenvironment, and gut microbiome in determining ICI response. Key factors include tumour mutation burden (TMB), neoantigens, and immune checkpoint molecules like PDL1. TMB is associated with ICI response, but other factors such as tumour heterogeneity, immune cell infiltration, and genetic mutations also play roles. PDL1 expression is a biomarker for ICI response, but its predictive value is limited. Tumour-infiltrating lymphocytes (TILs) and immune-inflamed, immune-excluded, and immune-desert phenotypes are also important in determining ICI outcomes. Understanding these factors is crucial for optimizing immunotherapy and improving patient outcomes.