Tight clusters of CD4+ T cells and CD20+ B cells containing PNAd+ high endothelial venules surrounded by CD138+ plasma cells were observed in skin lesions from 6 patients with PV, 3 patients with PF, and 1 patient with PNP (Figure 1A and Supplemental Figure 1, A–C; supplemental material available online with this article; https://doi.org/10.1172/JCI166357DS1). This evidence concerns the gene CD4 and acquired polycythemia vera.