diff --git a/nxontology/node.py b/nxontology/node.py
index c921b1d..0535f06 100644
--- a/nxontology/node.py
+++ b/nxontology/node.py
@@ -16,6 +16,7 @@
 # Type definitions. networkx does not declare types.
 # https://github.com/networkx/networkx/issues/3988#issuecomment-639969263
 NodeT = TypeVar("NodeT", bound=Hashable)
+NodeInfoT = TypeVar("NodeInfoT")
 
 
 class NodeInfo(Freezable, Generic[NodeT]):
@@ -35,7 +36,7 @@ class NodeInfo(Freezable, Generic[NodeT]):
     Each ic_metric has a scaled version accessible by adding a _scaled suffix.
     """
 
-    def __init__(self, nxo: NXOntology[NodeT], node: NodeT):
+    def __init__(self, nxo: NXOntology[NodeT, NodeInfoT], node: NodeT):
         if node not in nxo.graph:
             raise NodeNotFound(f"{node} not in graph.")
         self.nxo = nxo
diff --git a/nxontology/ontology.py b/nxontology/ontology.py
index 55c3d34..11c6e7d 100644
--- a/nxontology/ontology.py
+++ b/nxontology/ontology.py
@@ -3,6 +3,7 @@
 import itertools
 import json
 import logging
+from abc import abstractmethod
 from os import PathLike, fspath
 from typing import Any, Generic, Iterable, cast
 
@@ -12,7 +13,7 @@
 from networkx.algorithms.isolate import isolates
 from networkx.readwrite.json_graph import node_link_data, node_link_graph
 
-from nxontology.node import NodeT
+from nxontology.node import NodeInfoT, NodeT
 
 from .exceptions import DuplicateError, NodeNotFound
 from .node import NodeInfo
@@ -22,7 +23,7 @@
 logger = logging.getLogger(__name__)
 
 
-class NXOntology(Freezable, Generic[NodeT]):
+class NXOntologyBase(Freezable, Generic[NodeT, NodeInfoT]):
     """
     Encapsulate a networkx.DiGraph to represent an ontology.
     Regarding edge directionality, parent terms should point to child term.
@@ -77,7 +78,7 @@ def write_node_link_json(self, path: str | PathLike[str]) -> None:
             write_file.write("\n")  # json.dump does not include a trailing newline
 
     @classmethod
-    def read_node_link_json(cls, path: str | PathLike[str]) -> NXOntology[NodeT]:
+    def read_node_link_json(cls, path: str | PathLike[str]) -> NXOntologyBase[NodeT]:
         """
         Retrun a new graph from node-link format as written by `write_node_link_json`.
         """
@@ -213,7 +214,8 @@ def compute_similarities(
             yield metrics
 
     @classmethod
-    def _get_node_info_cls(cls) -> type[NodeInfo[NodeT]]:
+    @abstractmethod
+    def _get_node_info_cls(cls) -> type[NodeInfoT]:
         """
         Return the Node_Info class to use for this ontology.
         Subclasses can override this to use a custom Node_Info class.
@@ -222,7 +224,7 @@ def _get_node_info_cls(cls) -> type[NodeInfo[NodeT]]:
         """
         return NodeInfo
 
-    def node_info(self, node: NodeT) -> NodeInfo[NodeT]:
+    def node_info(self, node: NodeT) -> NodeInfoT:
         """
         Return Node_Info instance for `node`.
         If frozen, cache node info in `self._node_info_cache`.
@@ -306,3 +308,9 @@ def set_graph_attributes(
             self.graph.graph["node_identifier_attribute"] = node_identifier_attribute
         if node_url_attribute:
             self.graph.graph["node_url_attribute"] = node_url_attribute
+
+
+class NXOntology(NXOntologyBase[NodeT, NodeInfo[NodeT]]):
+    @classmethod
+    def _get_node_info_cls(cls) -> type[NodeInfo[NodeT]]:
+        return NodeInfo
diff --git a/nxontology/similarity.py b/nxontology/similarity.py
index cf88253..7cfe310 100644
--- a/nxontology/similarity.py
+++ b/nxontology/similarity.py
@@ -3,12 +3,14 @@
 import math
 from typing import TYPE_CHECKING, Any, Generic
 
+from nxontology.ontology import NXOntologyBase
+
 if TYPE_CHECKING:
-    from nxontology.ontology import NXOntology
+    pass
 
 from networkx import shortest_path_length
 
-from nxontology.node import NodeInfo, NodeT
+from nxontology.node import NodeInfo, NodeInfoT, NodeT
 from nxontology.utils import Freezable, cache_on_frozen
 
 
@@ -29,7 +31,9 @@ class Similarity(Freezable, Generic[NodeT]):
         "batet_log",
     ]
 
-    def __init__(self, nxo: NXOntology[NodeT], node_0: NodeT, node_1: NodeT):
+    def __init__(
+        self, nxo: NXOntologyBase[NodeT, NodeInfoT], node_0: NodeT, node_1: NodeT
+    ):
         self.nxo = nxo
         self.node_0 = node_0
         self.node_1 = node_1
@@ -125,7 +129,7 @@ class SimilarityIC(Similarity[NodeT]):
 
     def __init__(
         self,
-        nxo: NXOntology[NodeT],
+        nxo: NXOntologyBase[NodeT],
         node_0: NodeT,
         node_1: NodeT,
         ic_metric: str = "intrinsic_ic_sanchez",