The purpose of the study is to evaluate safety and tolerability of oral quercetin in reducing inflammation in male patients with XIAP deficiency. Quercetin is a naturally occurring antioxidant that has many properties including the ability to decrease inflammation in other diseases.

XIAP deficiency is a rare genetic immune disorder. XIAP usually suppresses inflammation and XIAP deficiency leads to increased inflammation. Patients may develop several inflammatory problems including hemophagocytic lymphohistiocytosis (HLH), a life-threatening syndrome of overwhelming inflammation characterized by fever, low blood counts (cytopenias), an enlarged spleen, liver problems, seizures, and other manifestations. Patients may develop milder symptoms such recurrent fevers or problems with low blood counts. Inflammatory bowel disease (IBD), arthritis or inflammation of the eyes (uveitis) can also occur, among other more rare complications. XIAP deficiency is often life-threatening and patients with severe disease require treatment with allogeneic hematopoietic cell transplantation (HCT), which is the only available curative treatment. Unfortunately, allogeneic HCT is associated with a high risk of mortality and significant side effects. Therefore, it's important to find other treatment options that are safe without major side effects.

The purpose of the study is to evaluate safety and tolerability of oral quercetin in reducing inflammation in male patients with XIAP deficiency. Quercetin is a naturally occurring antioxidant that has many properties including the ability to decrease inflammation in other diseases.

Inclusion Criteria:

• Male patients
• Diagnosis of XIAP deficiency
• Patients ≥2 years of age
• Serum IL-18 ≥600 pg/mL (approximately 25% above the lab-defined upper limit of normal of 477 pg/mL)
• Able to take enteral medication

Exclusion Criteria:

• Renal failure requiring dialysis
• Total bilirubin >3 mg/dl and/or SGPT >300 at time of enrollment
• Patients receiving digoxin therapy, who are unable to discontinue treatment due to medical reasons
• Patients receiving fluoroquinolone therapy, who are unable to discontinue treatment due to medical reasons
• Patients who are at risk of pregnancy or fathering a child and are unable to use acceptable methods of birth control during the length of the study
• Patients who have received quercetin or any over the counter anti-oxidant supplementation within last 1 month
• Patients with unstable disease status or other medical issues requiring hospitalization or rapid escalation of medical care
• Participating in another therapeutic study for XIAP deficiency