Protein kinase genes account for almost 10% of all currently known cancer genes, highlighting the role of signal transduction in oncogenesis. A reexamination of the literature and available databases shows that E3 ubiquitin ligases are also key mediators of tumorigenesis. Altogether kinase and E3 genes represent more than 15% of the known cancer genes, underlining the importance of phosphorylation and ubiquitylation signaling pathways in cancer formation. Considering the recent literature reporting correlations between alterations in ubiquitylation processes and oncogenesis, this percentage is likely to increase even further in the future. Finally, E3 genes could serve as baits for the identification of additional cancer genes (e.g. their interacting partners). In contrast, deubiquitinases, like phosphatases, are not overrepresented among cancer genes. The same holds for E1 and E2 genes. Thus, kinase and E3 genes represent primary targets as cancer susceptibility genes for mutation screening and for the design of novel therapies.