Brain biopsies from 2 patients with FANS. (A) H&E and immunohistochemical IHC stains of the brain biopsy of patient 1; (Ai) demonstrates a nonspecific necrotic-inflammatory reactive process with reactive astrocytes (arrows) and macrophages (arrowheads), 100×; (Aii) 200×; (Aiii) necrosis with minute flecks of mineralization (arrowheads). (Aiv-v) IHC staining of the brain biopsy of patient 1 with an antibody against SV40 T antigen that cross-reacts with JCV T antigen, demonstrating positive staining of scattered parenchymal cells (arrows). (vi) IHC staining of a positive control brain biopsy from a patient with progressive multifocal leukoencephalopathy (PML), showing more widespread staining of cells (arrows). (B) H&E and IHC stains of the brain biopsy of patient 2; (Bi-ii) abundant reactive appearing endothelium within many of the vessels, and necrosis, axonal spheroids, gliosis, abundant macrophages, and thick-walled hyalinized microvessels (arrows); (Biii) luminal occlusion (arrows); (Biv-v) IHC staining of the brain biopsy of patient 2 with an antibody against SV40 T antigen, demonstrating positive staining of scattered cells in the parenchyma (arrows) and the walls of blood vessels (arrowheads); and (Bvi) endothelial cells (arrowheads), more widespread than seen in patient 1. H&E, hematoxylin and eosin; IHC, immunohistochemical. JCV, JC virus. PML, progressive multifocal leukoencephalopathy. SV40, polyoma simian virus 40.