Results: By RT-PCR we showed expression of organic anion transporters 1 and 3, P-glycoprotein, ATP-binding cassette transporters 4 and 6. Expression of proximal tubule marker aquaporin 1 was demonstrated by western blotting. In light microscopy, hPTEC displayed characteristic cobblestone shapes. Using immunostaining, we demonstrated expression of the tight junction protein ZO-1. We found a maximum increase in methylmalonic acid and propionyl-carnitine concentrations in hPTEC of mut0 patients, and less pronounced elevations in mut-patients.