Fig 2: White matter lesions of vector‐treated patient with MPSIIIA show no unfolded protein response activation, but profound depletion of heparan sulfate. Immunohistochemical staining against the unfolder protein/integrated stress response marker 4EBP1 shows no positive cells in the control (A) and untreated MPSIIIA white matter (B). In the vector‐treated MPSIIIA patient lymphocytes express 4EBP1 (C). Strongly 4EBP1‐positive astrocytes in Vanishing White Matter (VWM) serve as control (D). Electron microscopy image of a control (E) shows normal ensheathed axons, while an image of the vector‐treated patient (F) shows several axons ensheathed by slightly thinned, though normally compacted myelin. On the left, the nucleus of an astrocyte is seen. No lysosomes are visible. Staining against sulfamidase (SGSH) shows normal enzyme expression in the cellular cytoplasm of control white matter (G), near depletion of enzyme immunoreactivity in the untreated MPSIIIA white matter (H), and enzyme overexpression in the vector‐treated MPSIIIA patient (I). Fluorescence immunostaining against the highly sulfated heparan sulfate motifs (HS‐High, green) combined with the astrocyte marker glial fibrillary acidic protein (GFAP, red) shows that, compared to control white matter (J) HS‐High is markedly accumulated in the white matter of untreated MPSIIIA patients (K), but virtually devoid in the vector‐treated MPSIIIA white matter (L). Original magnifications (A–D; G–I) 200× bar: 10 μm; (E,F) 18 500× bar: 1 μm (J–L) 400× bar: 5 μm.