Although Tofacitinib provides a good treatment for RA, it also causes several severe side effects, such as anaemia and neutropenia, which are possibly caused by the inhibition of JAK212–14; recent IC50 data highlight the fact that Tofacitinib is indeed not an isoform-selective JAK3 inhibitor15–20, which means Tofacitinib is currently considered and widely accepted as a pan-JAK inhibitor. Here, JAK3 is linked to rheumatoid arthritis.