The huge stretches of genetic material dismissed in biology classrooms for generations as “junk DNA” actually contain instructions essential for the growth and survival of people and other organisms, and may hold keys to understanding complex diseases like cancer, strokes and heart attacks, researchers reported today.

That is the most striking finding of the first comprehensive comparison between the genetic instruction set, or genome, of human beings and that of laboratory mice. The new results suggest that the genomes of both organisms contain at least twice as much critically important genetic material as previously believed, a finding that promises to upend decades of scientific dogma and rewrite the rule book for how nature builds complex creatures.