Somatic mutations activating STAT3 have been discovered especially in T-cell large granular lymphocytic (T-LGL) leukemias (40%) as well as in Felty syndrome (neutropenia, splenomegaly, rheumatoid arthritis) and less frequently in aplastic anemia and myelodysplastic syndrome [24–27]. This evidence concerns the gene STAT3 and rheumatoid arthritis.