A method of screening a candidate compound for susceptibility to biliary excretion by a hepatocyte transport protein, the method comprising: (a) establishing first and second cell cultures, each first and second culture comprising a plurality of hepatocytes and at least one bile canaliculus, wherein the first culture expresses a hepatocyte transport protein and the second culture is at least partially inhibited from expressing the hepatocyte transport protein; wherein inhibiting expression of the hepatocyte transport protein in the second culture comprises introducing an siRNA directed against a hepatocyte transport protein into at least one of the plurality of hepatocytes in an amount sufficient to inhibit expression of the hepatocyte transport protein, wherein the siRNA comprises a ribonucleotide sequence which corresponds to a coding strand of a gene encoding the hepatocyte transport protein; (b) exposing a candidate compound to the first culture and to the second culture for a time sufficient to allow uptake of the candidate compound; (c) washing and lysing the first and second cultures; and (d) determining an amount of the candidate compound present in a lysate obtained from each culture in step (c) and using the amount of the candidate compound in each culture lysate to evaluate the candidate compound for susceptibility to biliary excretion by the transport protein, wherein the presence of a reduced amount of the candidate compound in the at least one bile canaliculus of the first cell culture as compared to an amount of the candidate compound in the at least one bile canaliculus of the second cell culture indicates the susceptibility of the candidate compound to biliary excretion by the hepatocyte transport protein. (a) establishing first and second cell cultures, each first and second culture comprising a plurality of hepatocytes and at least one bile canaliculus, wherein the first culture expresses a hepatocyte transport protein and the second culture is at least partially inhibited from expressing the hepatocyte transport protein; wherein inhibiting expression of the hepatocyte transport protein in the second culture comprises introducing an siRNA directed against a hepatocyte transport protein into at least one of the plurality of hepatocytes in an amount sufficient to inhibit expression of the hepatocyte transport protein, wherein the siRNA comprises a ribonucleotide sequence which corresponds to a coding strand of a gene encoding the hepatocyte transport protein; (b) exposing a candidate compound to the first culture and to the second culture for a time sufficient to allow uptake of the candidate compound; (c) washing and lysing the first and second cultures; and (d) determining an amount of the candidate compound present in a lysate obtained from each culture in step (c) and using the amount of the candidate compound in each culture lysate to evaluate the candidate compound for susceptibility to biliary excretion by the transport protein, wherein the presence of a reduced amount of the candidate compound in the at least one bile canaliculus of the first cell culture as compared to an amount of the candidate compound in the at least one bile canaliculus of the second cell culture indicates the susceptibility of the candidate compound to biliary excretion by the hepatocyte transport protein.