Genetic studies of cardiac electrocardiographical parameters such as heart rate, atrioventricular conduction, ventricular depolarization, or repolarization have focused either on rare families with Mendelian inheritance of severe phenotypes (familial sinus bradycardia, conduction disease, or repolarization abnormalities such as long QT syndrome) or on large cohorts of ostensibly normal individuals assessing small interindividual differences in these electrocardiogram measurements. The earliest studies (1990s) assessed microsatellite markers (repeated sequences of 2–5 nucleotides) that have a higher rate of mutations and thus generate multiple polymorphisms (alleles) and occur every few thousand base pairs. Single nucleotide polymorphisms (SNPs) occur approximately every thousand base pairs but typically have only 2 alleles and low heterozygote frequencies. Nevertheless, SNP mapping has become a popular technique for both the analysis of segregation of a phenotype within families and the genome-wide association study (GWAS) within populations. To date, 85% of genetic mutations underlying inherited diseases occur within the 1% of genomic DNA that codes for proteins (the exome). These protein-coding genes typically also have a promoter region adjacent to and upstream of the gene (of variable length but typically in the range of 250 base pairs) that acts to initiate transcription of that gene. In addition, genes have enhancer regions that are regulatory DNA elements and are usually within several thousand base pairs of a gene (cis-effect) but may be distant by hundreds of thousands of base pairs, and even on a different chromosome (trans-effect). In addition to promoters and enhancers, other portions of noncoding DNA act as templates for transfer RNAs, ribosomal RNAs, and regulatory RNAs. Until recently, the large remaining proportion of noncoding DNA had no identified biological function and has been termed “junk DNA.”