Cytokine response assays. (A-B) Colony formation with rhSCF at 50 ng/mL and (A) rhG-CSF (Amgen) or (B) rhTPO, each at 100 ng/mL or 0.8 ng/mL. Results are from normal subjects (n = 6), JAK2V617I cases (n = 4), and JAK2V617F-positive MPNs (n = 6), expressed as numbers of colonies formed with 0.8 ng/mL rhG-CSF or rhTPO as a percentage of colony numbers formed with 100 ng/mL rhG-CSF or rhTPO. (C) The numbers of BFU-E generated with conditioned methylcellulose (see Methods) in the presence of rhEPO at 5 U/mL, 0.05 U/mL, or 0.005 U/mL, or in the absence of EPO. Results are expressed as a percentage relative to total numbers of BFU-Es generated at 5 U/mL for normal controls (n = 4), JAK2V617I cases (n = 4), and JAK2V717F cases (n = 7). P values are shown if < .1. Error bars represent SEM.