Example of CDK and history of their research will be used to discuss roles of kinases and other enzymes in regulation of basic biological processes and treatment of human diseases. We will discuss roles of CDKs in regulation of cell cycle, transcription and mRNA processing and how deregulation of these processes leads to onset of human diseases, particularly cancer. We will review CDKs as therapeutic targets and biomarkers in cancer and discuss challenges in bringing CDK inhibitors to successful clinical applications.