Prof Randy Lewis shows Adam Rutherford genetically modified goats at a farm at Utah State University, US, which produce large quantities of a spider silk that is among the strongest substances known to man.

The transplanted gene means the goat produce milk containing an extra protein, which is extracted and spun into spider silk thread.

Horizon: Playing God investigates the new science of synthetic biology on BBC Two at 21:30 GMT on Tuesday 17 January. Watch online (UK only) or see more clips at the above link.