diff --git a/lisp/mail/mu4e-configuration.el b/lisp/mail/mu4e-configuration.el
index 0ed5b03..5cc3555 100644
--- a/lisp/mail/mu4e-configuration.el
+++ b/lisp/mail/mu4e-configuration.el
@@ -167,7 +167,7 @@
                   (append mu4e~headers-thread-state '(t))))
         ;; Return the thread prefix.
         (format "%s%s"
-                prefix
+                (propertize prefix 'face 'mu4e-related-face)
                 (if duplicate
                     (mu4e~headers-thread-prefix-map 'duplicate) "")))))