We recently reported the isolation and partial characterization of two novel proteins, MAP17 and PDZK1. Using in situ hybridization, we demonstrated that MAP17 and PDZK1 mRNAs are markedly up-regulated in human carcinomas. PDZK1, originally isolated as a protein interacting with MAP17, contains four PDZ protein-interaction domains and could potentially interact with as many as four target proteins. In this paper, we confirm the overexpression of PDZK1 in human carcinomas using a specific antibody and demonstrate the localization of the PDZK1 gene to human chromosome 1q21, a region frequently altered in neoplastic conditions. Using the yeast two-hybrid system, we have also determined that PDZK1 interacts with the carboxy-terminal portion of cMOAT (MRP2), the canalicular multispecific organic anion transporter associated with multidrug resistance. This is of particular interest because proteins containing PDZ domains are involved in the clustering and signaling pathways of membrane-associated proteins, including ion channels. Therefore, the protein cluster formed by the association of cMOAT, PDZK1, and MAP17 could play an important role in the cellular mechanisms associated with multidrug resistance, and PDZK1 may represent a new target in cancer cells resistant to chemotherapeutic agents.