diff --git a/README.md b/README.md
index ab481020..1bddad76 100644
--- a/README.md
+++ b/README.md
@@ -38,7 +38,7 @@
 | 4.2.1 Conditionals | incomplete | TODO: `cond-expand` |
 | 4.2.2 Binding constructs | incomplete | TODO: `let-values`, `let*-values` |
 | 4.2.3 Sequencing | yes | |
-| 4.2.4 Iteration | no | |
+| 4.2.4 Iteration | yes | TODO: `do` is unsafe when nested |
 | 4.2.5 Delayed evaluation | N/A | |
 | 4.2.6 Dynamic bindings | no | TODO: `make-parameter`, `parameterize` in C level |
 | 4.2.7 Exception handling | no | `guard` syntax. |