The different physiological environments provided by the hosts probably generated a strong selective force for the evolution of differential expression of gene products in digenetic kinetoplastid parasites. Indeed, the sorts of molecules that would be predicted to be stage-regulated have been found to be differentially expressed. Examples include transport molecules, metabolic enzymes, molecules involved in cell adhesion and penetration, and molecules involved in evasion of host defence mechanisms. Within the life cycle, environmental cues provided by the host, such as temperature and nutrients, may also function as triggers for the differential expression of gene products.