We have recently discovered a new class of potassium channels with two pore-forming domains and four membrane-spanning domains. When heterologously expressed, these channels produce time- and voltage-independent currents that classify them as background or leak channels. TWIK (for tandem of P domains in a weak inwardly rectifying K+ channel) was the first member of this family to be cloned. Here, we describe the genomic organization of TWIK in the mouse. The coding sequence as well as the untranslated sequences are contained in three exons. The twik gene (or KCNK1) has been mapped to chromosome 8, consistent with its localization to 1q42-43 in human. The twik gene is expressed in virtually all mouse tissues. It is most abundantly expressed in brain and moderately in other organs such as kidney. The level of expression is increased in brain and kidney from neonate to adult animals, but the TWIK message is also detected during embryogenesis, as early as day 7 post conception.