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Grow your neuron as long as possible by clicking on protein targets.
You can only click on protein targets within your circle of influence but you need to be quick as it decreases over time.
Oh one more thing...beware of the rival red neurons that compete with you for available protein targets. Click on the special pink, green, yellow and blue protein targets for a range of power ups.
'Axon' uses real features of neuron development in the foetal brain. You can find out more by clicking on The Science link from the main menu of the game.