diff --git a/Tuesday/GREAT/GREAT_analysis_forEpigenomicData.md b/Tuesday/GREAT/GREAT_analysis_forEpigenomicData.md
index 63c5e89ffa943515ae97ec93eeddfecc893a322b..b2f3547ea8e02fd97c193fd1b0cfbad52e1b54e5 100644
--- a/Tuesday/GREAT/GREAT_analysis_forEpigenomicData.md
+++ b/Tuesday/GREAT/GREAT_analysis_forEpigenomicData.md
@@ -22,6 +22,13 @@ ChIPseq analyis done using hg19.
 </details>
 
 
+<details>
+  <summary markdown="span">Peaks have been called using the hg19 Human reference genome. Why is this information important?</summary>
+Because gene annotation, including gene location varies between assemblies.
+
+</details>
+
+
 ## Perfom analysis 
 
 * Go to http://great.stanford.edu/public/html/