Vector-based RNAi was used to establish a stable Caco-2 cell line with a persistent knockdown of multidrug resistant gene I (MDR1) and P-glycoprotein (P-gp). Several positive clones were collected, many of which showed significantly reduced levels of MDR1 mRNA and P-gyp compared to wt Caco-2 cells. Selected clones were sub-cultivated for six passages and real-time PCR showed that MDR1 expression remained significantly reduced (up to 96%) over this period of time. RNAi-MDR1 clones frozen long term also kept their low MDR1 expression levels when re-cultured. Permeability studies were performed across RNAi-MDR1 clone cell monolayers, and the efflux of cyclosporine A, digoxin, vinblastine, and vincristine showed 58%, 61%, 91%, and 78% decrease in active transport, respectively, compared to wt Caco-2 cells. This stably modified Caco-2 cell line provides a novel tool for studies on MDR1 and other ABC transporter protein gene Cellular functions. (C) 2004 Elsevier Inc. All rights reserved.