Complex sentence: Development in understanding cellular circuitry can lead to exciting new modifications, such as cells which can respond to environmental stimuli such as cells that signal toxic surroundings and react by activating pathways used to degrade the perceived toxin.
Split into simpler sentences: Development in understanding cellular circuitry can lead to exciting new modifications, such as cells which can respond to environmental stimuli. For example cells could be developed that signal toxic surroundings and react by activating pathways used to degrade the perceived toxin.