In a meticulous deep single-cell analysis of T cell receptor (TCR)- and RNA-sequencing from colorectal cancer patient biopsies, Zemin Zhang’s group found that patients with microsatellite-instable tumours (which show a strikingly favourable response profile to ICB therapy) showed preferential enrichment for a TH1-like subset of CD4+ T cells. Here, CD4 is linked to colorectal cancer.