diff --git a/Biobase/Secondary/Isostericity.hs b/Biobase/Secondary/Isostericity.hs
--- a/Biobase/Secondary/Isostericity.hs
+++ b/Biobase/Secondary/Isostericity.hs
@@ -15,6 +15,7 @@
 
 import Data.ByteString.Char8 (ByteString)
 import Data.FileEmbed (embedFile)
+import Data.Function (on)
 import Data.List
 import qualified Data.ByteString.Char8 as BS
 import qualified Data.Map as M
@@ -77,7 +78,8 @@
 
 -- | Process CSV list-of-lists to get the isostericity data.
 
-mkIsostericityList gs = concatMap f gs where
+mkIsostericityList :: [[[String]]] -> [(ExtPair, [String])]
+mkIsostericityList gs = nubBy ((==) `on` fst) . concatMap turn . concatMap f $ gs where
   f g = map (\e ->  ( ( let [x,y] = fst e
                         in (mkNuc x, mkNuc y), threeChar bpt
                       )
@@ -86,6 +88,7 @@
     bpt = head $ head g
     xs = tail g
     entry x = (x!!0, takeWhile ((=='I') . head) $ drop 2 x)
+  turn entry@(((x,y),(wc,tx,ty)), cs) = [entry, (((y,x),(wc,ty,tx)), cs)]
 
 -- | Simple parsing of raw CSV data.
 
diff --git a/BiobaseXNA.cabal b/BiobaseXNA.cabal
--- a/BiobaseXNA.cabal
+++ b/BiobaseXNA.cabal
@@ -1,5 +1,5 @@
 name:           BiobaseXNA
-version:        0.5.5.0
+version:        0.5.6.0
 author:         Christian Hoener zu Siederdissen
 maintainer:     choener@tbi.univie.ac.at
 homepage:       http://www.tbi.univie.ac.at/~choener/
@@ -26,9 +26,13 @@
                 .
                 Nucl. Acids Res. (2009)
                 .
-                doi:10.1093/nar/gkp011
+                <http://dx.crossref.org/10.1093%2Fnar%2Fgkp011>
                 .
                 .
+                .
+                Changes since 0.5.5.0
+                .
+                * isostericity for the 6 "missing" classes
                 .
                 Changes since 0.5.4.0
                 .