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New information about the biology behind the heart’s electrical activity has been revealed in a major genome study with the largest sample size ever of a project of this type. These molecular mechanisms offer insights into cardiac electrical diseases and could suggest avenues of drug research for preventing and treating heart rhythm or conduction problems. Genes in 44 loci identified as associated with the PR interval on an EKG are overrepresented in cardiac disease processes, including heart block, sick sinus syndrome, and atrial fibrillation.