SOCS3 enhances JAK2 V617F–induced proliferation. (A) Ba/F3 cells expressing EPOR (top graph), EPOR and wild-type JAK2 (middle graph), or EPOR and JAK2 V617F (bottom graph) in which SOCS3 was absent due to the presence of tetracycline (□) or in which expression of SOCS3 was induced by the removal of tetracycline (■) were incubated in the presence of 5 U/mL EPO. Viable cell numbers determined by trypan blue exclusion. Significant differences between samples at each time point are indicated: *P < .05, **P < .01. (B) Ba/F3 cells stably expressing JAK2 V617F alone, or in combination with EPOR and/or SOCS3 as indicated were incubated in the presence of 5 U/mL EPO (top panels) or in the absence of cytokine (bottom panel), and viable cell numbers determined by trypan blue exclusion (top and bottom panels) or MTT assay (middle panel). Significant differences of samples to JAK2 V617F + EPOR + SOCS3 sample at each time point are indicated: *P < .05, **P < .01. Error bars represent standard deviation (SD).