1. 
   Xaa
   Symbol for unspecified amino acid. ... (05 Mar 2000) ...
   Found op http://www.mondofacto.com/facts/dictionary?Xaa
   
   
2. 
   Xaa
   Type: Symbol Definitions: 1. Symbol for unspecified amino acid (also expressed as X).
   Found op http://www.medilexicon.com/medicaldictionary.php?t=100097
   
   
3. 
   XAA
   XAA is a three-letter abbreviation with multiple meanings, as described below: ...
   Found op http://en.wikipedia.org/wiki/XAA
   
   
4. 
   xaa
   1) Symbol for unspecified amino acid
   Found op http://www.mijnwoordenboek.nl/EN/crossword-dictionary/xaa/1
   
   

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