diff --git a/compact/definitions.xml b/compact/definitions.xml
index e6e086092340063d62dd7f44ae2e7343ddd3b5d9..ed853680ef327f26f331a60057d6b13e235963c0 100644
--- a/compact/definitions.xml
+++ b/compact/definitions.xml
@@ -122,6 +122,8 @@
     <constant name="SiTrackerEndcapP_Layer5_ID"     value="69"/>
     <constant name="SiTrackerEndcapN_Layer5_ID"     value="70"/>
 
+    <constant name="cb_CTD_Si_ID"                   value="74"/>
+
     <comment> 
     ===================
     (75-99 Reserved IDs