To comprehensively characterise the phenotypes of tumour-infiltrating cytotoxic T cells, Chou and colleagues started with single cell RNA sequencing analysis of CD45+TCRβ+CD8α+ cells directly isolated from the breast cancer tissues of MMTV-PyMT (PyMT) mice.1 Bioinformatic clustering identified five distinct T cell populations including naïve/recently activated, exhausted and proliferative cells, as well as αβILTCKs characterised by high NK1.1 expression.3 These cells are characterised by a unique transcriptome that is distinct from that of conventional tumour-infiltrating CD8+ T cells. The gene discussed is CD8A; the disease is breast cancer.