diff --git a/chebai/preprocessing/bin/protein_token/tokens.txt b/chebai/preprocessing/bin/protein_token/tokens.txt
index 72ad1b6d..c31c5b72 100644
--- a/chebai/preprocessing/bin/protein_token/tokens.txt
+++ b/chebai/preprocessing/bin/protein_token/tokens.txt
@@ -18,3 +18,4 @@ W
 E
 V
 H
+X