diff --git a/Data/Weighting/GSC.hs b/Data/Weighting/GSC.hs
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+module Data.Weighting.GSC where
+
+import Data.List (foldl')
+import Data.Clustering.Hierarchical (Dendrogram(..))
+
+-- | /O(n^2)/ Calculates the Gerstein\/Sonnhammer\/Chothia
+-- weights for all elements of a dendrogram.  Weights are
+-- annotated to the leafs of the dendrogram while distances in
+-- branches are kept unchanged.
+--
+-- Distances @d@ in branches should be non-increasing and between
+-- @0@ (in the leafs) and @1@.  The final weights are normalized
+-- to average to @1@ (i.e. sum to the number of sequences, the
+-- same they would sum if all weights were @1@).
+gsc :: Fractional d => Dendrogram d a -> Dendrogram d (a, d)
+gsc (Leaf x) = Leaf (x,1)
+gsc dendro   = let (wsum, nsum, r) = go wfinal undefined [] dendro
+                   wfinal = (wsum / fromIntegral nsum)
+               in r
+    where
+      position (Leaf _)       = 0 -- no difference from itself
+      position (Branch d _ _) = d
+
+      go wfinal _ cs (Branch d l r) =
+          let (wl, nl, l') = go wfinal d ((el / wl) : cs) l
+              (wr, nr, r') = go wfinal d ((er / wr) : cs) r
+
+              el = d - position l -- edge length to left branch
+              er = d - position r --          ...to right branch
+
+              wsum = wl + wr + el + er
+              nsum = nl + nr
+          in wsum `seq` nsum `seq` (wsum, nsum, Branch d l' r')
+      go wfinal d cs (Leaf x) =
+          -- O(n) worst case, O(log n) best case (balanced dendrogram)
+          let w = foldl' (\curw c -> curw + curw * c) d (tail cs)
+          in (0, 1 :: Int, Leaf (x, w / wfinal))
diff --git a/LICENSE b/LICENSE
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+++ b/LICENSE
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+Copyright (c)2010, Felipe Lessa
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Felipe Lessa nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/Setup.hs b/Setup.hs
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+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/gsc-weighting.cabal b/gsc-weighting.cabal
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+Name:                gsc-weighting
+Version:             0.1
+Synopsis:            Generic implementation of Gerstein/Sonnhammer/Chothia weighting.
+License:             BSD3
+License-file:        LICENSE
+Author:              Felipe Almeida Lessa
+Maintainer:          felipe.lessa@gmail.com
+Category:            Clustering
+Build-type:          Simple
+Cabal-version:       >= 1.6
+Description:
+  In their 1994 paper \"Volume Changes in Protein Evolution\",
+  Gerstein, Sonnhammer and Chothia developed a weighting
+  procedure for protein sequences to avoid over-represented
+  sequences in the appendix \"A Method to Weight Protein
+  Sequences to Correct for Unequal Representation\".  Although
+  their method was developed for protein sequences, it is readily
+  extended to work on any measurable set.
+  .
+  This package calculates GSC weights for any reasonable
+  dendrogram.  If you want to recreate their algorithm, then just
+  use @UPGMA@ as linkage and residue identity as distance
+  function when creating the dendrogram.
+
+Library
+  Exposed-modules:
+    Data.Weighting.GSC
+  Build-depends: base == 4.*, hierarchical-clustering == 0.*
+  GHC-options: -Wall