What is this summary about?: This is a plain language summary of an article originally published in Blood. Leniolisib is a drug developed to treat activated PI3K delta syndrome (APDS). APDS is a rare disease in which the immune system does not work correctly. People with APDS have a wide range of symptoms including infections, certain organs associated with the immune system becoming larger, and worse quality of life. This summary reports the results from a clinical study that aimed to understand if leniolisib is effective at treating people with APDS. What were the results?: The study showed that leniolisib improved the amounts of immune cells and antibodies (proteins that help the immune system to function). Leniolisib also reduced the size of participants' enlarged organs, and lowered the activity of the protein that causes APDS. There were no major safety concerns for participants who took leniolisib. What do the results mean?: These results indicate that leniolisib helps the immune system to work in a way that is closer to those without APDS. By helping the immune system, leniolisib may be able to improve the lives of people with APDS by making them feel better. Leniolisib may also prevent difficulties in completing activities in the daily lives of people with APDS. Clinical Trial Registration:NCT02435173 (ClinicalTrials.gov).