Our results demonstrate that (a) the Kir2.1 gene encodes a native K+ channel protein with a 21-pS conductance; (b) this channel has an important role in the genesis of adult ventricular 1K1; and (c) the contribution of Kir2.1 channel proteins to 1K1 changes during development. The lack of contribution of Kir2.1 to fetal 1K1 channels is interesting from the point of view of possible future generation of knockout mice lacking Kir2.1, since cardiac abnormalities would not be expected to result in fetal lethality. These observations provide further support for a generalized hypothesis that different genes may code for 1K1 channel proteins at various developmental stages. However, the effects of these AS-oligos must first be examined on native 1K1 channels in cardiac myocytes before definite conclusions can be reached.