CD274 and acquired polycythemia vera: The results of our study can explain, at least in part, the lack of efficacy (clinical or bone marrow pathologic response) of the pembrolizumab treatment in patients with advanced primary, post-ET-MF, and post-PV myelofibrosis [88] and shed more light on the relationship between the types of driver mutations, the PD-L1 expression, and the ET progression to the fibrotic phase.