Effects of vorinostat on cell proliferation and clonogenic outgrowth. (A) BA/F3-EpoR cells stably expressing JAK2V617F, HEL, and UKE-1 cells harboring the JAK2V617F mutation or BCR-ABL–transformed K562 cells expressing WT JAK2 were treated with various concentrations of vorinostat for 48 hours. Cell proliferation was measured with the WST assay. Columns represent the mean of 3 independent experiments; bars represent SEM. (B) Cells were treated with DMSO (control) or vorinostat, and viable cells were counted by trypan blue exclusion over 5 days. Results shown represent means ± SEM from 3 independent experiments. (C) BA/F3-EpoR-JAK2V617F, HEL, UKE-1, and K562 cells were left untreated or treated with vorinostat (0.5 or 1.0μM) and plated in methylcellulose medium (Methocult M3234) in the absence of cytokine. Cytokine-independent colonies were counted on day 5. Results shown represent means ± SEM from 3 independent experiments. Note that vorinostat significantly inhibited the clonogenic growth of JAK2V617F-transformed cells.