The KIR family of potassium channels possess a greater tendency to allow potassium to flow into the cell rather than out of it. Kir4.1, also known as Kir1.2, is highly expressed in brain including glial cells, astrocytes and cortical neurons. Kir4.1 is also expressed in myelin-synthesizing oligodendrocytes and is crucial to myelination in the developing nervous system. The gene encoding human Kir4.1 maps to chromosome 1. Kir4.2, also known as Kir1.3, is expressed in kidney, lung, heart, thymus and thyroid during development. The gene encoding human Kir4.2 maps to chromosome 21 in the Down syndrome chromosome region 1, and Kir4.2 may play a role in the pathogenesis of Down’s syndrome. Kir 5.1 forms functional channels only by coexpression with either Kir4.1 or Kir4.2 in the kidney and pancreas. The gene encoding human Kir5.1 maps to chromosome 17.