The Top 10 Everything of 2009

When it comes to understanding a disease as complex as Alzheimer's, the more the better — genes, that is. In September, 15 years since the last discovery of its kind, scientists finally identified a new set of genes that may contribute to the memory-robbing disorder. Two groups of researchers, working separately, homed in on three genes linked to the late-onset form of the disease, the type that hits people in their 60s or later and accounts for 90% of Alzheimer's cases in the U.S. Two of the genes are known to interact with the amyloid-protein plaques that build up in the brain of Alzheimer's patients and eventually cause nerve-cell death and cognitive problems. The third affects the junction of nerve cells, where various neurochemicals work to relay signals from one nerve cell to another. It's not clear yet exactly how the genes increase Alzheimer's risk — in fact, most healthy people have some version of the three genes — but researchers hope that the growing pool of genetic factors will eventually help them develop more effective and better-targeted treatments for the disease.