Phosphatidyl inositol phosphates (PIPs) are universal markers of intracellular membranes. PI(4)P is the single most abundant mono phosphoinositide that defines the membranes of the Golgi and TGN. It has been reported to play a key role in membrane biogenesis, vesicular transport, lipid dynamics, and protein and lipid sorting in the TGN. Importantly, the replication of several RNA viruses, mainly the Hepatitis C virus (HCV), depends on PI(4)P. The goal of this project is to get molecular insight in the PI(4)P biosynthesis that takes place at intracellular membranes of all eucaryotic organisms. We have solved the crystal structures of the kinase domains of the PI(4)Ks. The structural data enable us to design specific inhibitors that could be used as antiviral agents.