The mRNA encoding full-length erythropoietin (EPO) receptor (EPOR-F) comprises exons I through VIII. Another membrane-bound EPOR (EPOR-T) isoform has a truncated cytoplasmic region and is encoded by the mRNA containing unspliced intron VII (EPOR-T mRNA). EPOR-T is believed to have a dominantly negative function against EPOR-F. We show that EPOR-T mRNA is markedly decreased in the blood cells of patients with polycythemia vera (PV). We also show that EPOR-T mRNA is not detected in erythroid/megakaryocytic leukemia cell lines, but is expressed in nonerythroid/nonmegakaryocytic lines, suggesting the presence of a cell type-specific system by which intron VII of the EPOR transcript is spliced. Deregulation of this splicing system in early hematopoietic progenitors possibly explains the profound decrease in EPOR-T mRNA and consequent pathophysiology of PV.