These directories contain the downloadable tables in the UCSC UniProt database.
This database is shared by tracks that incorporate UniProt data, such as the
UCSC Genes tracks. For more information on UniProt, see http://www.uniprot.org/.

These tables are updated periodically.  The directories are named according to
the date of download from UniProt, with the format: spYYMMDD.

To see descriptions of the tables in uniProt, visit the Table Browser:
  http://genome.ucsc.edu/cgi-bin/hgTables
select "All Tables" as the group, select uniProt as the database,
and select a table.  Then click the "describe table schema" button.

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UniProt's License and Disclaimer:
http://www.uniprot.org/help/license

Credits:

The UniProt Consortium. Reorganizing the protein space at the Universal Protein
Resource (UniProt). Nucleic Acids Res. 2012 Jan 1;40(D1):D71-D75. Epub 2011 Nov 18.
http://dx.doi.org/doi:10.1093/nar/gkr981