Malaria parasite (Plasmodium berghei). These P. berghei ookinetes (elongated) and unfertilised eggs (round) have been taken from the midgut of a mosquito (Anopheles sp.). Ookinetes are zygotes, an early stage in the life cycle of the parasite. They are glowing green under ultraviolet light because a gene for green fluorescent protein (GFP) has been introduced into their genome from the jellyfish Aequorea victoria. The green glow shows that the GFP gene has been successfully introduced. P. berghei causes malaria in mice, but it cannot cause disease in humans and its similarity to human malaria-causing protozoa makes it a good model organism for malaria research.