 You could think of this exercise we're going through as trying to take a computer apart, to understand how it works. Imagine that you were trying to understand a computer that you've seen for the first time, where you have to disassemble it in order to think about how this thing might actually function. First you take it apart, you sort its parts, you characterize them, and then you try to put it back together again. This is exactly what we're trying to do with the brain. The parts are really the cells. There are billions of cells in the brain, and we're trying to group them based on their similarity. We're trying to build a sort of periodic table of all of the cells that are present in the human cortex. The way that we're trying to classify the cells is to look at the genes that they express, and to group them together based on similarity in that gene expression. Establishing this parts list will allow us to be able to understand the molecular machinery so that we can use that knowledge to design new drug treatments. In neuroscience, we've spent a lot of time and energy trying to understand how the brain produces behavior in what we call model organisms. Most commonly these days we use mice, but when we try to take that knowledge and turn it into new treatments for people, sometimes it works, but a lot of the time it doesn't. We're trying to do the equivalent of what Ancestry.com or 23andMe are doing. We're doing genetic genealogy of cells. The idea is that just like one can measure the DNA in your own genome, we can measure the genetic content of individual cells, and then we can look at shared similarity across different cells on the basis of your genes. In this comparison between mouse and human cell types, we found that the basic architecture has been conserved over about 75 million years. But at the same time, we found that many of those types have changed dramatically, and this study allowed us to match those cell types between the species and then understand what are the differences between those types. One of the striking differences between mouse and human was in the receptors for serotonin, which is known to be involved in depression, anxiety. In both species, these proteins are expressed, but they're expressed in different kinds of cells, which should put a certain amount of doubt into the use of the mouse as a model organism to study things that affect serotonin signaling. The eventual goal will be to develop that complete parts list and map of the entire human brain. It's an ambitious goal. We're talking about billions of neurons. We're talking about trillions of connections. It's a huge undertaking, but what this paper shows is it's a doable undertaking. Ten years ago, I would have thought this was impossible. Today, I think it's inevitable. The technology is there, the need is there, and I think that with the public investment through initiatives like NIH's Brain Initiative, this is certainly going to become a reality in the future.