The phosphorylation of STAT1, STAT3, and STAT5 on the activating residues was measured and compared in three subsets of immune cells (CD3+ T cells, CD19+ B cells, and CD14+ monocytes) in 20 SLE patients, 9 RA patients and 13 HDs (Figure 1A). Here, STAT1 is linked to systemic lupus erythematosus.