Crosstide is a peptide analog of glycogen synthase kinase α/β fusion protein sequence which is a substrate for Akt. In Vitro: 16HBE14o− cells exposed to RV39 demonstrate a crosstide kinase activity in vitro. Serine phosphorylation of crosstide is confirmed by immunoblotting, and phosphorylation is blocked by PP2 but not by PP3^[1]. The wildtype GST-AKT2 shows significant phosphoryl transferase activity towards crosstide, reaching an initial velocity of 16 pmol phosphate/min/μg enzyme. The mutant GST-AKT2T/E,S/D displayed an initial velocity of 85 pmol phosphate/min/μg kinase for phosphorylation of Crosstide, corresponding to a 5-fold increase compared to the wildtype enzyme^[2].