diff --git a/modules/io/doc/mmcif.rst b/modules/io/doc/mmcif.rst
index ccde8f725d0a72e1151d674f5adc16ad1f792068..5d96ba6b85227b1408f892842e37818011b612ac 100644
--- a/modules/io/doc/mmcif.rst
+++ b/modules/io/doc/mmcif.rst
@@ -873,7 +873,8 @@ of the annotation available.
 
    .. attribute:: first_release
 
-   Index of the revision releasing this entry.
+   Index + 1 of the revision releasing this entry. If the value is 0, was not
+   set, yet.
 
    :type: :class:`int`