There are three main technical challenges associated with the morphology analysis: generally higher GFAP levels in PD which might skew the results of the analysis (for example, if finer branches are more clearly labelled in PD but are not visible in controls due to lower GFAP), the depth of the slices (5 μm), which does not capture the whole astrocytic process tree, and potential morphological heterogeneity among astrocytic subtypes87, which may lead to uneven subtype selection across individuals due to the lack of subtype biomarkers. The gene discussed is GFAP; the disease is Parkinson disease.