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.

Link to Allen Cell Types Database Hub

They were initiated in September 2003 with a $100 million donation from Paul G. Allen. As of May 2012, seven brain atlases have been published: Mouse Brain Atlas, Human Brain Atlas, Developing Mouse Brain Atlas, Developing Human Brain Atlas, Mouse Connectivity Atlas, Non-Human Primate Atlas, and Mouse Spinal Cord Atlas.