Liu: BridgeBio Pharma: Employment, Equity Ownership. Gibson:EUSA Pharma: Employment. Kanhai:EUSA Pharma: Employment. Martin:EUSA Pharma: Employment. Srkalovic:Takeda: Speakers Bureau; Janssen: Speakers Bureau; Foundation Medicine: Speakers Bureau; EUSA Pharma: Speakers Bureau. Uldrick:Patent: Patents & Royalties: co-inventor on US Patent 10,001,483 entitled ; Celgene: Other: research support from Celgene through a CRADA at the NCI; Roche: Other: commercial research support through a CTA with Fred Hutchinson Cancer Research Center; Merck: Other: drug for a clinical trial from Merck through a CRADA with the NCI. van Rhee:Takeda: Consultancy; Sanofi Genzyme: Consultancy; Karyopharm Therapeutics: Consultancy; EUSA: Consultancy; Adicet Bio: Consultancy; Kite Pharma: Consultancy; Castleman Disease Collaborative Network: Consultancy. Fajgenbaum:Janssen Pharmaceuticals: Research Funding.Tocilizumab, a monoclonal antibody directed against IL-6-receptor, is approved for use in rheumatoid arthritis in the US. It is frequently used off-label in idiopathic multicentric Castleman disease (iMCD) and is recommended as a substitute first-line therapy in the International Consensus iMCD treatment guidelines. Rituximab, a monoclonal antibody directed against CD20, is used in rheumatoid arthritis and other autoimmune and cancerous disorders. It is frequently used off-label in iMCD and is recommended as an alternate first-line or a second-line therapy in the International Consensus iMCD treatment guidelines. Corticosteroids are used broadly in iMCD and are recommended as needed as useful adjunctive therapy in the International Consensus iMCD treatment guidelines.