diff --git a/ontology/observable/observable.ttl b/ontology/observable/observable.ttl
index 9d141ecd..44c66234 100644
--- a/ontology/observable/observable.ttl
+++ b/ontology/observable/observable.ttl
@@ -3824,6 +3824,12 @@ observable:MessageFacet
 			sh:nodeKind sh:BlankNodeOrIRI ;
 			sh:path observable:to ;
 		] ,
+		[
+			sh:datatype xsd:boolean ;
+			sh:maxCount "1"^^xsd:integer ;
+			sh:nodeKind sh:Literal ;
+			sh:path observable:isRead ;
+		] ,
 		[
 			sh:datatype xsd:dateTime ;
 			sh:maxCount "1"^^xsd:integer ;