diff --git a/chebi_utils/obo_extractor.py b/chebi_utils/obo_extractor.py
index 1fc427f..fd427ca 100644
--- a/chebi_utils/obo_extractor.py
+++ b/chebi_utils/obo_extractor.py
@@ -63,7 +63,7 @@ def _term_data(doc: "fastobo.term.TermFrame") -> dict | None:
     }
 
 
-def build_chebi_graph(filepath: str | Path) -> nx.DiGraph:
+def build_chebi_graph(filepath: str | Path, top_class: str | None = "23367") -> nx.DiGraph:
     """Parse a ChEBI OBO file and build a directed graph of ontology terms.
 
     ``xref:`` lines are stripped before parsing as they can cause fastobo
@@ -82,6 +82,12 @@ def build_chebi_graph(filepath: str | Path) -> nx.DiGraph:
     ----------
     filepath : str or Path
         Path to the ChEBI OBO file.
+    top_class : str or None
+        CHEBI ID of the top-class (default "23367" for "molecular entity").
+        This will only return direct or indirect subclasses of the
+        top-class (excluding the top-class). If ``top_class`` is not
+        present in the parsed graph, the full graph is returned.
+        If None, the full graph is returned without subgraph extraction.
 
     Returns
     -------
@@ -113,7 +119,16 @@ def build_chebi_graph(filepath: str | Path) -> nx.DiGraph:
             for part_id in parts:
                 graph.add_edge(node_id, part_id, relation=relation)
 
-    return graph
+    if top_class is None:
+        return graph
+
+    hierarchy = get_hierarchy_subgraph(graph)
+    if top_class not in hierarchy:
+        return graph
+
+    chebi_subgraph = graph.subgraph(nx.ancestors(hierarchy, top_class))
+    assert isinstance(chebi_subgraph, nx.DiGraph)
+    return chebi_subgraph
 
 
 def get_hierarchy_subgraph(chebi_graph: nx.DiGraph) -> nx.DiGraph:
@@ -122,3 +137,14 @@ def get_hierarchy_subgraph(chebi_graph: nx.DiGraph) -> nx.DiGraph:
     return chebi_graph.edge_subgraph(
         (u, v) for u, v, d in chebi_graph.edges(data=True) if d.get("relation") == "is_a"
     )
+
+
+if __name__ == "__main__":
+    import argparse
+
+    parser = argparse.ArgumentParser(description="Build a ChEBI graph from an OBO file.")
+    parser.add_argument("obo_file", type=Path, help="Path to the ChEBI OBO file.")
+    args = parser.parse_args()
+
+    graph = build_chebi_graph(args.obo_file)
+    print(f"Final graph: {len(graph.nodes)} nodes, {len(graph.edges)} edges")