PRRC2B and acquired polycythemia vera: Both class I and II HLA genes are down‐regulated in PV, ET and myelofibrosis, with progressive downregulation of certain genes [BAT2L, HLA Complex Group 11 (HCG11) and major histocompatibility complex (MHC), Class I‐related (MR1)] in ET versus PV versus myelofibrosis.27