A protein kinase substrate is a molecule or structure that is capable of fitting into a specific catalytic active site of the kinase. A protein kinase substrate is phosphorylated by a protein kinase. These substrates can be either endogenous or exogenous. Exogenous substrates are used during in vitro kinase assay experiments to determine kinase activity. These substrates may be synthetic peptides, or recombinant proteins. The phosphorylation of the substrate protein changes its activity, stability, localization, or protein-protein interactions. This in turn causes changes in cell signaling and different cellular processes.