The species "Trypanosoma brucei", which causes the disease, has often been subdivided into three subspecies that were identified based either on the vertebrate hosts which the strain could infect or on the virulence of the disease in humans. The trypanosomes infectious to animals and not to humans were named "Trypanosoma brucei brucei". Strains that infected humans were divided into two subspecies based on their different virulences: "Trypanosoma brucei gambiense" was thought to have a slower onset and "Trypanosoma brucei rhodesiense" refers to strains with a more rapid, virulent onset. This characterization has always been problematic but was the best that could be done given the knowledge of the time and the tools available for identification. A recent molecular study using restriction fragment length polymorphism analysis suggests that the three subspecies are polyphyletic, so the elucidation of the strains of "T. brucei" infective to humans requires a more complex explanation. Procyclins are proteins developed in the surface coating of trypanosomes whilst in their tsetse fly vector.