In Western blot analyses the monoclonal antibody mESL reacts selectively with the 90 kD protein (after SDS-PAGE) present in the basolateral membrane fractions of human liver. According to a recent new nomenclature this hepatocytespecific protein is identical to OATP1B1 also known as OATP-C. Using immunofluorescence microscopy mESL stains the sinusoidal (basolateral) membranes in frozen sections of human liver, hepatocellular carcinoma and the human cell line HEK293 transfected with SLC21A6.