The phoshatidylinositol-3 kinase (PI3K) can be activated by binding directly or via the protein IRS-1 to the activated receptor tyrosine kinase. Alternatively, PI3K can be activated by binding to the GTP-bound form of the membrane bound protein Ras. Active PI3K catalyzes the formation of membrane bound PIP3. The second messenger PIP3 activates the serine-threonine kinase AKT. Active AKT phosphorylates many proteins, which leads to the inhibition of apoptosis and to the activation of translation and proliferation.