F Zhang et al. defined distinct subsets of CD8+ T cells characterized by GZMK+, GZMB+, and GNLY + phenotypes in RA joint synovial tissues by integrating single-cell transcriptomics and mass cytometry (Zhang et al., 2019), indicating an important role of hub gene GZMB in the classification of CD8+ T cells in RA. The gene discussed is GZMB; the disease is rheumatoid arthritis.