INCB018424 potently and selectively inhibits JAK2V617F-mediated signaling and proliferation. Ba/F3-EpoR-JAK2V617F (A) or HEL (B) cells were treated with increasing concentrations of INCB018424 for 2.5 hours, and extracts from these cells were subjected to immunoblot analysis for phosphorylated (p) or total forms of proteins associated with the JAK/STAT signaling pathway. JAK1 phosphorylation was assessed in primary MF patient samples (C) by immunobloting after immunoprecipitation of total JAK1. The numbers below describe the mutant JAK2V617F allele burden in patient samples. The effect of INCB018424 on viable cell number (D) was assessed after a 48-hour treatment period in cells expressing JAK2V617F (Ba/F3-EpoR-JAK2V617F and HEL) or, to confirm the selective nature of INCB018424, BCR-ABL (TF-1-BCR-ABL and Ba/F3-BCR-ABL).