Many of the protein kinase activators and inhibitors presently under investigation are natural products or are derivatives of natural products. Since over 500 kinases are encoded by the human genome, the task is to discover modulatory compounds which have high specificity for a single enzyme. Many different low molecular weight compounds have been studied for activity. These come from various sources, such as plants, marine organisms, microorganisms and cyanobacteria. The search for specific inhibitors is mainly performed in cell-based assays.