A group of researchers has taken a new approach to studying human brain development: they are growing and using organoids, three-dimensional millimeter-sized tissues that can be grown from what are called pluripotent stem cells. Research on organoids made of human cellular material has the advantage that the results are transferable to humans. In a new study just published in Nature, its researchers have now studied thousands of individual cells in a brain organoid at different times and in great detail. The goal of this study was to systematically identify genetic switches that have a significant impact on the development of neurons in different regions of brain organoids. Using a CRISPR-Cas9 system, the researchers selectively turned off one gene in each cell, for a total of about two dozen genes simultaneously throughout the organoid. This allowed them to discover what role the respective genes played in the development of the brain organoid.