diff --git a/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java b/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java
index bff92ec..1aba2eb 100644
--- a/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java
+++ b/src/main/java/eu/dnetlib/dhp/schema/common/ModelConstants.java
@@ -22,6 +22,7 @@ public class ModelConstants {
 	public static final String DNET_PROVENANCE_ACTIONS = "dnet:provenanceActions";
 	public static final String DNET_COUNTRY_TYPE = "dnet:countries";
 	public static final String DNET_REVIEW_LEVELS = "dnet:review_levels";
+	public static final String DNET_EXTERNAL_REFERENCE_TYPE = "dnet:externalReference_typologies";
 
 	public static final String SYSIMPORT_CROSSWALK_REPOSITORY = "sysimport:crosswalk:repository";
 	public static final String SYSIMPORT_CROSSWALK_ENTITYREGISTRY = "sysimport:crosswalk:entityregistry";