PIK3R1 and Down syndrome: Activated phosphoinositide 3-kinase δ syndrome (APDS), also known as PASLI disease (p110d-activating mutation causing senescent T cells, lymphadenopathy, and immunodeficiency) is a combined immunodeficiency resulting from dominant, gain-of-function mutations in the genes encoding p110δ (PIK3CD) and p85α (PIK3R1), the catalytic and regulatory subunits of phosphoinositide 3-kinase δ (PI3Kδ).