Go about 145,000,000 bases (or "letters") down the long arm of chromosome 1 and you'll come to 1q21.1, the genetic address of a small but important piece of DNA that is particularly prone to mistakes. When chromosome 1 is duplicated during normal cell division (say, in creating sperm or eggs), short, repetitive bits of DNA within this stretch are all too likely to mistakenly pair up, raising the chances that the new cells will have extra or missing copies of specific pieces of DNA.

Those small mistakes can have a big impact on people who carry them. Several studies in the last year have found that missing or extra pieces of DNA in the 1q21.1 region put the bearer at risk for a surprisingly broad range of psychiatric and neurological disorders, including autism, schizophrenia, and mental retardation. The discovery that one piece of DNA can lead to such diverse outcomes is opening new avenues in the study of disease. Rather than focusing solely on finding a common genetic flaw in everyone with a particular disease, researchers have begun to examine the various consequences that the same genetic flaw may have in different people. These studies suggest that even patients with different diagnoses may share common biological problems. "It's been eye-opening," says Mark Daly, a geneticist at the Broad Institute in Cambridge, MA, "because it's made us realize that in searching for the molecular basis of disease, it may be profitable to search for connections between seemingly unrelated phenotypes." Last year, Daly and his colleagues identified a section of DNA on chromosome 16 that also raises the risk of several different brain disorders, suggesting that this pattern may be common in the genetics of disease.