Together, these data indicate that PI3Kδ inhibition causes profound changes in the tumour microenvironment (TME), characterized by enhanced CD4+ and CD8+ T cell activation, oligoclonal T cell expansion and increased cytolytic activity, consistent with a decrease in intratumoural Treg cells, enabling T cell activation and leading to a rapid onset of dose-limiting toxicity. This evidence concerns the gene CD8A and neoplasm.