diff --git a/CHANGELOG.md b/CHANGELOG.md
index 101d405987..132278f2d6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -342,7 +342,9 @@ Note: 4.1.0-beta.0 release also contains all the changes present in the 4.0.1 re
 * **router:** should pass new data to Observable when query params change ([#15387](https://github.com/angular/angular/issues/15387)) ([08f2f08](https://github.com/angular/angular/commit/08f2f08)), closes [#15290](https://github.com/angular/angular/issues/15290)
 * prevent strictNullChecks support until [#15432](https://github.com/angular/angular/issues/15432) is fixed ([#15434](https://github.com/angular/angular/issues/15434)) ([b800a0c](https://github.com/angular/angular/commit/b800a0c))
 
+### BREAKING CHANGES
 
+From 4.0.0 @angular/core uses a [`WeakMap`](https://github.com/angular/angular/commit/52b21275f4c2c26c46627f5648b41a33bb5c8283), a polyfill needs to be included for [browsers that do not support it natively](http://kangax.github.io/compat-table/es6/#test-WeakMap).
 
 <a name="4.0.0-rc.6"></a>
 # [4.0.0-rc.6](https://github.com/angular/angular/compare/4.0.0-rc.5...4.0.0-rc.6) (2017-03-23)