However, seropositive RA synovial tissue and fluid samples contained a large population of PD-1hi CXCR5− T cells, comprising ~25% of the CD4+ T cells, that expressed high levels of IL-21 and CXCL13 and induced memory B cell differentiation into plasma cells in vitro. The gene discussed is CXCL13; the disease is rheumatoid arthritis.