Mouse stem cells switch on a neural developmental program when the activity of a specific gene lasts for a certain length of time.
Cells are flooded with many signals from gene expression. To find out how cells pick out the important ones from the background noise, Matt Thomson and his colleagues at the University of California, San Francisco, engineered mouse embryonic stem cells so that the Brn2 gene turned on when it was exposed to light. When the activity of this gene reached a specific duration and level, the stem cells rapidly began specializing into neural progenitor cells.
Mathematical modelling showed that a positive feedback network in the Brn2 circuitry helps to ensure that the Brn2 signal rises above the noise.