Cdk9 is a serine-threonine cdc2-related kinase and its activity is not cell cycle-regulated. Cdk9 function depends on its kinase activity and also on its regulatory units: the T-family cyclins and cyclin K. Recently, several studies confirmed the role of cdk9 in different cellular processes such as signal transduction, basal transcription, HIV-Tat- and MyoD-mediated transcription and differentiation. All the referred data strongly support the concept of a multifunctional protein kinase with specific cytoplasmic and nuclear functions.