How does a single cell become a complex organism? The fascination and challenge of this question, says Hazel Sive, “drives me out of bed each day, makes me work long hours and keeps me excited about coming here.” Sive’s Whitehead lab investigates developing embryos for clues about how cells organize and form tissues and organs. Not only must an embryo determine what kinds of cells to grow, it must also place them in precise patterns, along three dimensions. As the embryo develops, cells signal to each other to move to a specific position, or a regulatory protein sends a command for a cell to align itself in a certain way. Sive’s particularly interested in the evolution of brain structure. Zebrafish serve as her model. The tiny, transparent embryos of this fish enable her to “look directly into the brain in a noninvasive way.” Sive plays a video showing how a brain forms from a sheet of cells that first rolls up into a tube. Then, three distinct areas emerge that correspond to key neural functions. If a gene responsible for this patterning of early brain structure goes awry, then the zebra fish embryo may emerge with a brain cavity that’s too big or too small to permit the intricate folding of neural tissue. This early process transpires the same way in all brains, whether zebrafish or human, says Sive. So study of mutant zebra fish brains and the genes that play a role in abnormal development shed light on brain malfunction in humans, including autism and mental retardation.