Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Protein | Natural Host | Normal Function | Prion State | Prion Phenotype | Year Identified |
| --- | --- | --- | --- | --- | --- |
| Ure2p | Saccharomyces cerevisiae | Nitrogen catabolite repressor | [URE3] | Growth on poor nitrogen sources | 1994 |
| Sup35p | Saccharomyces cerevisiae | Translation termination factor | [PSI+] | Increased levels of nonsense suppression | 1994 |
| HET-S | Podospora anserina | Regulates heterokaryon incompatibility | [Het-s] | Heterokaryon formation between incompatible strains | 1997 |
| vacuolar protease B | Saccharomyces cerevisiae | death in stationary phase, failure in meiosis | [β] | failure to degrade cellular proteins under N starvation | 2003 |
| MAP kinases | Podospora anserina | increased pigment, slow growth | [C] |  | 2006 |
| Rnq1p | Saccharomyces cerevisiae | Protein template factor | [RNQ+],[PIN+] | Promotes aggregation of other prions | 2008 |
| Mca1* | Saccharomyces cerevisiae | Putative Yeast Caspase | [MCA+] | Unknown | 2008 |
| Swi1 | Saccharomyces cerevisiae | Chromatin remodeling | [SWI+] | Poor growth on some carbon sources | 2008 |
| Cyc8 | Saccharomyces cerevisiae | Transcriptional repressor | [OCT+] | Transcriptional derepression of multiple genes | 2009 |
| Mot3 | Saccharomyces cerevisiae | Nuclear transcription factor | [MOT3+] | Transcriptional derepression of anaerobic genes | 2009 |
| Pma1+Std1 | Saccharomyces cerevisiae | Pma1 = major plasma membrane proton pump, Std1=minor pump | [GAR+] | Resistant to glucose-associated repression | 2009 |
| Sfp1 | Saccharomyces cerevisiae | Global transcriptional regulator | [ISP+] | Antisuppressor of certain sup35 mutations | 2010 |
| Mod5 | Saccharomyces cerevisiae |  | [MOD+] |  | 2012 |
Question:
how many prions have been characterized?
Answer:
13