Colored brain scan of a 17-year-old boy with mad cow disease. The bright yellow spots are a sign that the thalamus is damaged by diseased proteins. Simon Fraser/Science Source hide caption

itoggle caption Simon Fraser/Science Source

Sequencing the genes of a cancer cell turns up lots of genetic mutations — but some of them are harmless. The goal is to figure out which mutations are the troublemakers. Kevin Curtis/Science Source hide caption

itoggle caption Kevin Curtis/Science Source

To sleep, perchance to consolidate important connections in far-flung parts of the brain. Alberto Ruggieri/Illustration Works/Corbis hide caption

itoggle caption Alberto Ruggieri/Illustration Works/Corbis

Some researchers recommend starting mammogram screening at age 40, while others say age 50. Some doctors think screening should be based on a woman's overall risk for breast cancer, not just her age. Hero Images/Corbis hide caption

itoggle caption Hero Images/Corbis

The same nerve receptor that responds to the green paste on your sushi plate is activated by car exhaust, the smoke of a wildfire, tear gas and other chemical irritants. iStockphoto hide caption

itoggle caption iStockphoto