 Electron microscopy reveals that the tumor cells forming the Flexner–Wintersteiner rosette have ultrastructural features of primitive photoreceptor cells.Furthermore, the rosette lumen shows similar staining patterns as in rods and cones, suggesting that Flexner–Wintersteiner rosettes represent a specific form of retinal differentiation.In addition to being a characteristic finding in retinoblastomas, Flexner–Wintersteiner rosettes may also be found in pinealoblastomas and medulloepitheliomas.