PIK3R1 encodes the regulatory subunit p85α of phosphoinositide 3-kinase (PI3K), functioning as a critical adapter protein in insulin and growth factor signaling 1. PIK3R1 binds activated receptor tyrosine kinases through its SH2 domain and mediates recruitment of the p110 catalytic subunit to the plasma membrane, where PI3K phosphorylates PIP2 to generate PIP3, activating downstream AKT signaling essential for glucose uptake and glycogen synthesis 1. Beyond metabolic functions, PIK3R1 plays roles in receptor signaling (FGFR, KIT, PDGFR) and modulates ER stress responses via XBP1 nuclear translocation. Functionally, PIK3R1 stabilizes p110 and serves as an adapter to interact with insulin receptor substrates and growth factor receptors 1. Disease relevance includes mutations causing Activated PI3Kδ Syndrome type 2 (APDS2), characterized by immunodeficiency with recurrent infections, lymphoproliferation (70.4% of patients), hyper-IgM syndrome (48.1%), and autoimmunity 2. PIK3R1 mutations also associate with neonatal diabetes and appear in T-cell acute lymphoblastic leukemia as adverse prognostic indicators 34. Clinically, gain-of-function PIK3R1 variants are treatable with leniolisib, an FDA-approved PI3Kδ inhibitor, though some hotspots show partial drug resistance addressable by combination therapies 5. PIK3R1 hotspot mutations in colorectal cancers predict aspirin responsiveness, reducing recurrence risk 6.