There are billions of neurons in the human brain, and scientists want to know how they are connected. Cold Spring Harbor Laboratory (CSHL) Alle Davis and Maxine Harrison Professor of Neurosciences Anthony Zador, and colleagues Xiaoyin Chen and Yu-Chi Sun, published a new technique in Nature Neuroscience for figuring out connections using genetic tags. Their technique, called BARseq2, labels brain cells with short RNA sequences called “barcodes,” allowing the researchers to trace thousands of brain circuits simultaneously.