A group of scientists, including several at Harvard, have dived deeper into the mammalian brain than ever before by categorizing and mapping at the molecular level all of its thousands of different cell types. This complexity is why true understanding of many brain functions, including molecular mechanisms that underlie neurological diseases, remains so elusive. To create the first molecularly defined cell atlas of the whole mouse brain, a team led by Harvard’s Xiaowei Zhuang identified and spatially mapped thousands of unique cell types, most of which had never previously been characterized. “We identified 5,000 transcriptionally distinct cell populations,” said Zhuang, the David B. Arnold Professor of Science and a Howard Hughes medical investigator. Someday the basic outlines of the atlas could be applied to the human brain, 1,000 times larger than the mouse brain.