JAK2 and rheumatoid arthritis: The two most likely mechanistic hypotheses which could explain observed clinical reductions in PBNC in RA subjects by CP-690,550 are: 1) suppression of bone marrow myeloid progenitor cell differentiation via a loss of selectivity and inhibition of JAK2; and 2) suppression of chemotactic and hematopoietic growth factors as an extension of the anti-inflammatory activity of CP-690,550.