 - Diagnosis of Erdheim-Chester Disease by either 1 documented histopathologic establishing ECD or 2 documented histopathologic findings compatible with Erdheim-Chester disease in the context of corroborating clinical andor radiologic findings or 3 documented radiologic findings, in the absence of a biopsy having been performed or yielding evaluable tissue, that are felt by the Principal Investigator to unequivocally represent an ECD diagnosis given the clinical context. 