| Recruiting | N/A | 300 | Europe, Canada, US, RoW | Registry study | The Hospital for Sick Children | Rare Histiocytic Disorders (RHDs), Juvenile Xanthogranuloma (JXG), Reticulohistiocytoma (epithelioid Histiocytoma), Xanthoma Disseminatum (XD), Multicentric Reticulohistiocytosis (MRH), Systemic Juvenile Xanthogranuloma, Erdheim-Chester Disease (ECD), Multi-system Rosai-Dorfman Disease (RDD) | 09/28 | 09/28 | | |