The average human has about 37.2 trillion cells. But cells are differentiated for thousands of different functions. How does a cell ‘know’ which function to fulfill? In a paper published today in Developmental Cell, the group of Carl-Philipp Heisenberg at the Institute of Science and Technology Austria (IST Austria), including first author and Ph.D. student Vanessa Barone, sheds light on how a cell’s fate is determined. Ffor the first time, they report a positive feedback loop between the duration of cell-cell contacts and the specification of a cell’s function.