diff --git a/minimap2.1 b/minimap2.1
index a23147e..6943b8e 100644
--- a/minimap2.1
+++ b/minimap2.1
@@ -777,6 +777,7 @@ cs	Z	Difference string
 dv	f	Approximate per-base sequence divergence
 de	f	Gap-compressed per-base sequence divergence
 rl	i	Length of query regions harboring repetitive seeds
+zd	i	Alignment broken due to Z-drop; bit 1: left broken; bit 2: right broken
 .TE
 
 .PP