diff --git a/progress.md b/progress.md
index b218525..631875b 100644
--- a/progress.md
+++ b/progress.md
@@ -105,7 +105,7 @@
  - [x] 12.1 Why Reflection?
  - [x] 12.2 reflect.Type and reflect.Value
  - [x] 12.3 Display, a Recursive Value Printer
- - [ ] 12.4 Example: Encoding S-Expressions
+ - [x] 12.4 Example: Encoding S-Expressions
  - [ ] 12.5 Setting Variables with reflect.Value
  - [ ] 12.6 Example: Decoding S-Expressions
  - [ ] 12.7 Accessing Struct Field Tags