diff --git a/macs2.wdl b/macs2.wdl
index 7b11c99fed1065412897bda243826b3f282f76e4..8d89f3af89dbd7f2e88798c84b840816ac832a4b 100644
--- a/macs2.wdl
+++ b/macs2.wdl
@@ -32,7 +32,7 @@ task PeakCalling {
         Boolean? nomodel
         String? gensz
         Int? extsize
-        Int? shiftsize = -1*round(extsize/2)
+        Int? shiftsize
         Float? pval_thres
         Boolean? bdg
         String? keepdup