JAK1 and Arthritis: In in vitro hematopoiesis assays, CP-690,550 had no effect on human myeloid progenitor cell differentiation at concentrations which fully inhibit cytokine signaling pathways through JAK1 and JAK3 but do not appear to inhibit JAK2 in whole blood (Table 1), and are fully efficacious in rodent arthritis models and in human RA patients (Table 2, Figure 2) [17].