diff --git a/ML/AutoFocus.md b/ML/AutoFocus.md
index bf8c056..e3ec816 100644
--- a/ML/AutoFocus.md
+++ b/ML/AutoFocus.md
@@ -2,6 +2,7 @@
 
 Implement a CNN as described in this paper.
 
+https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5943362/
 
 Four focus stack images are available in the data folder for training 
 and testing.