diff --git a/genedescriptions/data_manager.py b/genedescriptions/data_manager.py
index 2e4f806..dc624f6 100644
--- a/genedescriptions/data_manager.py
+++ b/genedescriptions/data_manager.py
@@ -199,12 +199,12 @@ def set_ontology(self, ontology_type: DataType, ontology: Ontology, config: Gene
             else:
                 self.expression_ontology = ontology
         module = get_module_from_data_type(ontology_type)
-        ontology = self.get_ontology(data_type=ontology_type)
+        ontology: Ontology = self.get_ontology(data_type=ontology_type)
         terms_replacement_regex = config.get_module_property(module=module, prop=ConfigModuleProperty.RENAME_TERMS)
         if terms_replacement_regex:
             self.rename_ontology_terms(ontology=ontology, terms_replacement_regex=terms_replacement_regex)
         root_nodes = [n for n in ontology.nodes() if len(
-            list(ontology.predecessors(n))) == 0 and len(list(ontology.successors(n))) > 0]
+            list(ontology.parents(n))) == 0 and len(list(ontology.children(n))) > 0]
         set_all_depths(ontology=ontology, root_node_ids=root_nodes, relations=self.get_relations(ontology_type))
         if config.get_module_property(module=module,
                                       prop=ConfigModuleProperty.TRIMMING_ALGORITHM) == "ic":