The aim of the current study was to characterize the potency and selectivity of CP-690,550 for the JAK family members and to determine if PBNC reductions in the context of arthritis are related to the anti-inflammatory efficacy of CP-690,550 (through JAK 1 and JAK3 inhibition), or due to inhibition of hematopoiesis through inhibition of JAK2 at efficacious exposures. Here, JAK2 is linked to Arthritis.