Study of the ITK Inhibitor Soquelitinib to Reduce Lymphoproliferation and Improve Cytopenias in Autoimmune Lymphoproliferative Syndrome (ALPS)-FAS Patients

Background: Autoimmune lymphoproliferative syndrome (ALPS) is a rare disorder of the immune system caused by a mutation in the FAS gene. In ALPS, the body stores too many germ-fighting cells called lymphocytes. This can lead to an enlarged spleen and lymph nodes. Current treatments for ALPS can have many adverse effects. Better treatments for ALPS are needed. Objective: To test a study drug (soquelitinib) in people with ALPS. Eligibility: People aged 16 years and older with ALPS. Design: Participants will have 8 clinic visits and 6 remote visits within 1 year. Participants will be screened. They will have a physical exam with blood and urine tests. Some may have tests of their lung function. Soquelitinib is a tablet taken by mouth twice a day. Participants will record their doses and any symptoms on a paper or online form. Blood tests and other procedures will be repeated during study visits. Three visits will include imaging scans. Participants will lie on a table that slides through a doughnut-shaped machine while X-rays capture pictures of the inside of their body. Some participants may be able to remain in the study for a second year.

* INCLUSION CRITERIA: To be eligible to participate in this study and to re-enroll, an individual must meet all the following criteria: 1. Aged \>= 16 years. 2. Able to provide informed consent (for ages \>= 18 years) or has a parent(s) or guardian(s) who can provide permission to participate on their behalf (for ages \1.5 mg/dL (or 133 micromol/L) or estimated glomerular filtration rate \300 copies/mL. 6. Tuberculosis infection (active or latent) or undergoing tuberculosis treatment. 7. Infection with HIV or hepatitis B or C. 8. Current other invasive or systemic fungal, bacterial, or viral infection requiring therapy. 9. History of opportunistic infection within the previous 180 days. 10. History of invasive malignancy that required systemic therapy within the last 3 years. 11. Current use of moderate or strong cytochrome P450 isozyme (CYP)3A inhibitors or inducers that cannot be stopped before day 0. 12. Current use of P-glycoprotein (P-gp) inhibitors that cannot be stopped before day 0. 13. Known hypersensitivity or contraindication to CT contrast agent. 14. Pregnant or breastfeeding. 15. Any condition that, in the opinion of the study team contraindicates participation in this study.