Dictyostelium is a powerful model system in which to study developmental decision making, as illustrated by two new papers that report that DimB, a bZIP transcription factor, directly regulates the responses of Dictyostelium amoebae to the differentiation factor DIF-1. On starvation, Dictyostelium amoebae aggregate and form a migrating slug. During this process, they differentiate into prestalk (pst) or prespore cells. pstA cells occupy the tip of the slug, while pstO cells lie behind the tip and prespore cells occupy the rear four-fifths. DIF-1, which is made by prespore cells, is required for the differentiation of pstO cells, but its signal transduction pathway is largely unknown. Now, two groups report that DimB, a bZIP transcription factor, directly regulates DIF-1 responses in Dictyostelium. Zhukovskaya et al. identified DimB by purifying molecules that interact with two promoter elements in ecmA, a gene expressed in prestalk cells (see p. 439). They show that DimB establishes a gradient of ecmA expression in the slug tip by repressing its expression in cells at the rear and centre of the prestalk zone, and suggest that competition between DimB and an unknown activator controls ecmA expression. They also show that DimB accumulates in the nucleus when cells are exposed to DIF-1 and becomes associated with the ecmA promoter. Huang et al. used bioinformatics to identify DimB (see p. 449). Their search of the Dictyostelium genome for factors that could heterodimerize with DimA – another bZIP transcription factor that regulates Dictyostelium responses to DIF-1 – identified DimB. They show that DimB interacts with DimA in vitro and that DIF-1 stimulation of cells causes the rapid nuclear accumulation of both DimA and DimB. Together, these papers provide new insights into how DIF-1 controls Dictyosteliumdifferentiation and draw parallels with mammalian systems, where interactions between transcription factors increase their regulatory potential.