Unlike PI3Kα and PI3Kβ, which are ubiquitously expressed, PI3Kẟ and PI3Kγ are expressed primarily in cells of hematopoietic origin. The central role of PI3Kẟ in regulating numerous cellular functions of the adaptive immune system (B-cells and, to a lesser extent, T cells) as well as the innate immune system (neutrophils, mast cells, and macrophages) strongly indicates that PI3Kẟ is a valid and potentially effective therapeutic target for several immune diseases. To date, leniolisib has been well tolerated during both the Phase 1 first-in-human trial in healthy subjects and the Phase II/III registration-enabling study.