The serum of children with untreated hemophagocytic syndromes contains elevated levels (23,600 to 75,200 U/mL) of soluble interleukin-2 receptor (SIL2R) that returns toward normal with clinical improvement. These levels are in excess of levels previously reported for benign conditions. They are as high as levels reported for HTLV-1-associated adult T-cell leukemia (HATL) and hairy cell leukemia (HCL) in adults and some children with poor-prognosis non-T, non-B, acute lymphoblastic leukemia (ALL). Serum SIL-2R is a marker of disease activity that has the potential to identify infants at risk for the inherited form of the disease before the disease is clinically expressed.

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