Immunoprofiling of synovial CD4+ T cells of anti-citrullinated protein antibodies (ACPA)+ and ACPA− RA patients using the single cell sequencing technology identified specific GPR56 expression in the CXCL13high peripheral helper T (TPH) cell subset that contains most clonally expanded T cells in the inflamed joints [129]. Here, ADGRG1 is linked to rheumatoid arthritis.