HOXB13 and subependymoma: Abbreviations: MEPN =  myxopapillary ependymoma; spinal EPN = spinal ependymoma; SEPN = subependymoma; EPN ST = supratentorial ependymoma; EPN IF = infratentorial ependymoma; HOXB13 = homeobox B13; NEFL =  neurofilament light; PDGFRα = platelet‐derived growth factor receptor alpha.