PIK3R1 (PI3-kinase p85α, phosphoinositide-3-kinase regulatory subunit alpha) belongs to PI3K regulatory-subunit family and assembles PI3K holoenzyme for RTK-downstream signal transduction. As regulatory component of PI3K heterodimer, p85α harbors SH2 domains that bind phosphorylated tyrosine residues on activated receptor tyrosine kinases. It recruits p110 catalytic subunit to plasma membrane to trigger PI3K activity, converting PIP2 into PIP3 and initiating AKT signaling. Free p85α exerts inhibitory effect on p110 to keep pathway quiescent. Mutation and dysregulation of p85α are linked to malignancy and immune disorders.