STXBP2 and progressive multifocal leukoencephalopathy: Two genes (C8B and STXBP2) with PML risk variants are among the 437 genes designated by the International Union of Immunological Societies (IUIS) to cause inborn errors of immunity (43, 44), thereby supporting our original hypothesis (25) of host genetics as an additonal risk factor for development of PML.