diff --git a/ontobio/ontol.py b/ontobio/ontol.py
index 191d003d..5296fac7 100644
--- a/ontobio/ontol.py
+++ b/ontobio/ontol.py
@@ -598,12 +598,49 @@ def logical_definitions(self, nid):
         else:
             return []
 
+    def definition(self, nid):
+        """
+        Text definition object for a node
+
+        Arguments
+        ---------
+        nid : str
+            Node identifier for entity to be queried
+
+        Return
+        ------
+        dict
+            definition object, dict(val=TEXT, xrefs=LIST)
+        """
+        return self._get_meta_prop(nid, 'definition')
+        
+    def definition_val(self, nid):
+        """
+        Text definition string value for a node
+
+        Arguments
+        ---------
+        nid : str
+            Node identifier for entity to be queried
+
+        Return
+        ------
+        str
+            text definition
+        """
+        defn = self.definition(nid)
+        if defn is None:
+            return None
+        else:
+            return defn['val']
+    
+
     def get_node_type(self, nid):
         n = self.node(nid)
         if 'type' in n:
             return n['type']
         return None
-        
+ 
     def _get_meta_prop(self, nid, prop):
         n = self.node(nid)
         if 'meta' in n:
@@ -763,7 +800,7 @@ def xrefs(self, nid, bidirectional=False):
         nid : str
             Node identifier for entity to be queried
         bidirection : bool
-            If True, include nodes xreffed to nid
+            If True, include nodes that xref nid
 
         Return
         ------
@@ -773,7 +810,7 @@ def xrefs(self, nid, bidirectional=False):
             xg = self.xref_graph
             if nid not in xg:
                 return []
-            if bidirectional:
+            elif bidirectional:
                 return xg.neighbors(nid)
             else:
                 return [x for x in xg.neighbors(nid) if xg[nid][x]['source'] == nid]
diff --git a/ontobio/sim.py b/ontobio/sim.py
new file mode 100644
index 00000000..70cef000
--- /dev/null
+++ b/ontobio/sim.py
@@ -0,0 +1,105 @@
+import math
+import pandas as pd
+
+class SimEngine():
+
+    def __init__(self,
+                 association_set=None,
+                 icmap=None):
+        self.association_set = association_set
+        self.icmap = icmap
+
+    def _get_icmap(self):
+        if self.icmap is None:
+            icmap = {}
+            aset = self.association_set
+            num_subjs = len(asset.subjects)
+            for n in aset.ontology.nodes():
+                num_anns = len(aset.query([n]))
+                freq = num_anns / num_subjs
+                ic = None
+                if freq > 0:
+                    ic = -math.log(freq/num_subjs) / math.log(2)
+                icmap[n] = ic
+            self.icmap = icmap
+        return self.icmap
+        
+    def information_content(self,nid):
+        """
+        Returns information content for a node
+        """
+        icmap = self._get_icmap()
+        return icmap[nid]
+        
+    def entity_jaccard_similarity(self,s1,s2):
+        """
+        Calculate jaccard index of inferred associations of two subjects
+
+        |ancs(s1) /\ ancs(s2)|
+        ---
+        |ancs(s1) \/ ancs(s2)|
+
+        """
+        a1 = self.association_set.inferred_types(s1)
+        a2 = self.association_set.inferred_types(s2)
+        num_union = len(a1.union(a2))
+        if num_union == 0:
+            return 0.0
+        return len(a1.intersection(a2)) / num_union
+    
+    def class_jaccard_similarity(self,c1,c2):
+        """
+        Calculate jaccard index of two classes
+
+        |ancs(c1) /\ ancs(c2)|
+        ---
+        |ancs(c1) \/ ancs(c2)|
+
+        """
+        ont = self.association_set.ontology
+        a1 = ont.ancestors(c1,reflexive=True)
+        a2 = ont.ancestors(c2,reflexive=True)
+        num_union = len(a1.union(a2))
+        if num_union == 0:
+            return 0.0
+        return len(a1.intersection(a2)) / num_union
+
+    def class_resnik_similarity(self,c1,c2):
+        """
+        Calculate resnik similarty of two classes
+
+        Return
+        ------
+        (number,list)
+            tuple of max_ic and list of MRCAs
+        """
+        cas = self.common_ancestors(c1,c2)
+        pairs = [(a, self.information_content(a)) for a in cas]
+        max_ic = 0
+        mrcas = []
+        for a,ic in pairs:
+            if ic > max_ic:
+                max_ic = ic
+                mrcas = [a]
+            elif ic == max_ic:
+                mrcas.append(a)
+        return max_ic, mrcas
+    
+    def used_classes(self):
+        aset = self.association_set
+        cset = set()
+        for s in aset.subjects:
+            cset.update(aset.inferred_types(s))
+        return cset
+        
+    def dataframe(self):
+        aset = self.association_set
+        entries = []
+        subjs = aset.subjects
+        for s in subjs:
+            vmap = {}
+            for c in aset.inferred_types(s):
+                vmap[c] = 1
+            entries.append(vmap)
+        df = pd.DataFrame(entries, index=subjs)
+        return df
diff --git a/tests/test_local_json.py b/tests/test_local_json.py
index 613ef1ef..5a92cf25 100644
--- a/tests/test_local_json.py
+++ b/tests/test_local_json.py
@@ -142,6 +142,11 @@ def test_graph():
     assert NIF_CELL in xrefs
     assert len(xrefs) == 2
 
+    def_val = ont.definition_val(CELL)
+    assert def_val.startswith("The basic structural and functional unit of all organisms")
+
+    defn = ont.definition(CELL)
+    assert defn['xrefs'] == [ "GOC:go_curators" ]
     
     # xrefs are bidirectional
     xrefs = ont.xrefs(WIKIPEDIA_CELL, bidirectional=True)
diff --git a/tests/test_sim.py b/tests/test_sim.py
new file mode 100644
index 00000000..791bb849
--- /dev/null
+++ b/tests/test_sim.py
@@ -0,0 +1,34 @@
+from ontobio.ontol_factory import OntologyFactory
+from ontobio.assoc_factory import AssociationSetFactory
+from ontobio.assocmodel import AssociationSet
+from ontobio.io.gafparser import GafParser
+from ontobio.sim import SimEngine
+import logging
+import random
+
+
+
+POMBASE = "tests/resources/truncated-pombase.gaf"
+INTRACELLULAR='GO:0005622'
+G1 = 'PomBase:SPBC902.04'
+def test_sim():
+    """
+    Test loading from gaf
+    """
+    ofactory = OntologyFactory()
+    afactory = AssociationSetFactory()
+    ont = ofactory.create('tests/resources/go-truncated-pombase.json')
+    aset = afactory.create_from_gaf(open(POMBASE,"r"),
+                                    ontology=ont)
+
+    sim = SimEngine(aset)
+    for g1 in aset.subjects:
+        print("G1={} '{}'".format(g1, aset.label(g1)))
+        for g2 in aset.subjects:
+            print("  G2={} '{}'".format(g2, aset.label(g2)))
+            jsim = sim.entity_jaccard_similarity(g1,g2)
+            print("    SIM={}".format(jsim))
+
+    
+
+