CD4 and neoplasm: The expansion of TILs from multiregional biopsy samples, from distinct cancerous lesions within the same patient or from different regions within the same tumour, could give a more accurate snapshot of intratumour heterogeneity at a single time-point (Fig. 1b) and, therefore, enable the more successful design of TIL therapies targeting a substantial proportion of tumour cells in any given cancer.55 Neoantigen-specific CD8+ and CD4+ lymphocytes have also been detected in peripheral blood,56–58 which could overcome the problem of limited specimen availability in certain tumours.