Sections of DNA molecules that do not code for proteins but still perform important functions. Apparently, some are enhancers or suppressors of genes. About a fourth of DNA consists of introns. The remaining "junk" DNA, that makes up three fourths of a DNA molecule, very likely also has subtle functions. For instance, It helps determine the shape of chromosomes and also buffers against change by absorbing the mutagenic effect of radiation and viruses. See exons.