Phosphatidylinositides generated by PI3K recruit phosphatidylinositide-dependent protein kinase 1 (PDK1) and AKT (also known as protein kinase B) to the membrane, through their PH (pleckstrin-homology) domains. The binding of PIP3 to the PH domain of AKT is the rate-limiting step in AKT activation. In mammals there are three AKT isoforms (AKT1-3) encoded by three separate genes. The three isoforms share a high degree of amino acid identity and have indistinguishable substrate specificity in vitro. However, isoform-preferred substrates in vivo cannot be ruled out. The relative expression of the three isoforms differs in different mammalian tissues: AKT1 is the predominant isoform in the majority of tissues, AKT2 is the predominant isoform in insulin-responsive tissues, and AKT3 is the predominant isoform in brain and testes. All 3 isoforms are expressed in human and mouse platelets (Yin et al. 2008; O'Brien et al. 2008). Note: all data in the pathway refer to AKT1, which is the most studied.