Protein phosphatase 5 (PP5) possesses unique biochemical properties, which include its tetratricopeptide repeat (TPR) targeting/regulatory domain and its ability to he activated by lipids. PP5 has been studied as a paradigm for TPR protein structure and function. Roles for PP5 in signal transduction are emerging:from cell cycle regulation and signaling by nuclear receptors, to possible regulation of membrane receptors and ion channels.