Crosstide is the peptide counterpart of glycogen synthase kinase-3 (GSK-3). It functions as a natural substrate for Akt/PKB. Recognized by MAPKAP kinase-1 and p70S6K, this peptide also displays similar specificity towards PKBα, PKBβ, and PKBγ isoforms. The fluorescent and biotinlyated version of this peptide demonstrate similar enzymatic activities.