In a new preprint led by Jonathan and Anupam, we have developed a model of the Mecom—Cdk6 toggle switch, predicted to control early hematopoietic stem cell fate decisions as stem cells exit from quiescence and differentiate into a multipotent progenitor state. This two-gene circuit permits up to four coexisting stable steady states (tetrastability), including two intermediate states. Thus, we predict that stem-to-multipotent differentiation proceeds through a series of fine-scale transitions rather than a single commitment event. More broadly, analysis of this toggle simple two-gene circuit with complicated dynamics allowed us to address a deeper question that regarding that is fundamentally dynamical rather than molecular: how does the structure of such a small gene regulatory network determine the geometry of the landscape on which cell fate decisions are made, thereby controlling the paths that connect cell states. Read the preprint here.