Although GFAP was ubiquitously expressed in 84% (21/25) of ependymoma samples across all subtypes (Table S3), IF staining of FFPE slices confirmed preferential expression of GFAP (Fig. 4B vs. C/D) and ISG15 (Fig. 4F vs. G/H) in SP-SE. Here, GFAP is linked to ependymoma.