The human body contains hundreds of cell types originating through differentiation of multipotent precursors. The process of cellular differentiation relies on a series of important choices, from deciding whether to proliferate or exit the mitotic cycle, to committing to a specific cellular fate. At the molecular level, differentiation requires large-scale changes in the gene expression program. The precursor-specific genes have to be down-regulated whereas batteries of “specialist” genes have to be turned on in differentiation-committed cells. A remarkable aspect of these transitions is that they occur in a highly coordinated manner. We want to understand the mechanisms underlying this synchrony.