“The Allen Cell Types Database, on its surface, doesn’t look like much. The first release includes information on just 240 neurons out of hundreds of thousands in the mouse visual cortex, with a focus on the electrophysiology of those individual cells: the electrical pulses that tell a neuron to fire, initiating a pattern of neural activation that results in perception and action. But understanding those single cells well enough to put them into larger categories will be crucial to understanding the brain as a whole—much like the periodic table was necessary to establish basic chemical principles.”