Prion diseases or transmissible spongiform encephalopathies are fatal 
neurodegenerative diseases characterized by the aggregation and deposition of 
the misfolded prion protein in the brain. α-synuclein (α-syn)-associated 
multiple system atrophy has been recently shown to be caused by a bona fide 
α-syn prion strain. Several other misfolded native proteins such as β-amyloid, 
tau and TDP-43 share some aspects of prions although none of them is shown to be 
transmissible in nature or in experimental animals. However, these prion-like 
"prionoids" are causal to a variety of neurodegenerative diseases such as 
Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. The 
remarkable recent discovery of at least two new α-syn prion strains and their 
transmissibility in transgenic mice and in vitro cell models raises a distinct 
question as to whether some specific strain of other prionoids could have the 
capability of disease transmission in a manner similar to prions. In this 
overview, we briefly describe human and other mammalian prion diseases and 
comment on certain similarities between prion and prionoid and the possibility 
of prion-like transmissibility of some prionoid strains.