In order to investigate the ability of ultra-deep TCR sequencing to quantify intratumoural T cells, we stained formalin-fixed and paraffin-embedded (FFPE) sections obtained from tumour tissue adjacent to the fresh-frozen samples for the T cell markers CD4 (characteristic of T helper cells), CD8 (characteristic of cytotoxic T cells) and Foxp3 (characteristic of regulatory T cells). The gene discussed is CD4; the disease is neoplasm.