sentences:  This potential ability is suppressed by the inward-rectifier potassium current Ik1 encoded by the gene Kir2 which is not expressed in pacemaker cells.By specific inhibition of Ik1 below a certain level, spontaneous activity of cardiomyocytes was observed with resemblance to the action potential pattern of genuine pacemaker cells.Meanwhile, other genes and cells have been discovered, including heart muscle cells derived from embryonic stem cells, "HCN" genes which encode the wild type pacemaker current I(f).