Basic hypotheses concerning nature of an infectious agent of the transmissible spongiform encephalopathies are reported here. The agent may consist of a protein only (PrPsc), and for such a molecular structure the term "prion" has been coined. It may consist of a yet-to-be-discovered small oligonucleotide and a shell protein and this is the "virino" concept. Or, the agent may be a virus which has still eluded detection. The experiments with transgenic and knock-out mice proved that PrPsc is crucial for pathogenesis of spongiform encephalopathies. However, presence of strains of scrapie agent strongly favour a nucleic acid as a template of genetic information, which is difficult to reconcile with the "protein only" hypothesis. On the other hand, the development of spontaneous neurodegenerative disorder in Tg(GSS MoPrP) mice, if independently confirmed, strongly supports the "prion" hypothesis. Furthermore, the linkage of mutations within the PRNP gene with phenotypic appearance of Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker syndrome points to importance of the PrP gene. In conclusion, it must be said that the molecular structure of the agent is still uncertain.