diff --git a/Wolf-tutorial-with-the-OBITools3.md b/Wolf-tutorial-with-the-OBITools3.md
index 48ef398..cc8f0f4 100644
--- a/Wolf-tutorial-with-the-OBITools3.md
+++ b/Wolf-tutorial-with-the-OBITools3.md
@@ -140,7 +140,7 @@ For EMBL files, you can give the path to a directory with several EMBL files.
 		obi grep --require-rank=family --taxonomy wolf/taxonomy/my_tax wolf/v05_refs_uniq wolf/v05_refs_uniq_clean
 
 
-4. Build the reference database specifically used by the OBITools3 to make ecotag efficient:
+4. Build the reference database specifically used by the OBITools3 to make ecotag efficient by pre-computing the similarities between the reference sequences:
 
 		obi build_ref_db -t 0.97 --taxonomy wolf/taxonomy/my_tax wolf/v05_refs_uniq_clean wolf/v05_db_97