Generic peptide substrate for use in TR-FRET biochemical kinase assays. Refer to Selector Guide for validated kinases. Crosstide peptide (sequence: CGSGSGRPRTSSFAEG) labeled with ULight™ dye. Synthetic peptide derived from glycogen synthase kinase-3 alpha (GSK-3α) (Phosphorylation site: Ser21). GSK-3α can be phosphorylated by a variety of kinases including AKT1, AKT2, AKT3, MSK1, AMS2, PDK1, PLK3.