diff --git a/README.md b/README.md
index c9659c4e..63936e12 100644
--- a/README.md
+++ b/README.md
@@ -152,6 +152,23 @@ For example: `phparkitect debug:expression ResideInOneOfTheseNamespaces App`
 
 Currently, you can check if a class:
 
+### Is referenced in a given map
+
+This is useful, for example, to ensure that DTOs like commands and
+events are always set in a map, so that we are sure a serializer knows how
+to serialize/deserialize them.
+
+```php
+$map = [
+    'a' => 'App\Core\Component\MyComponent\Command\MyCommand',
+    'b' => 'App\Core\Component\MyComponent\Event\MyEvent',
+];
+$rules = Rule::allClasses()
+    ->that(new ResideInOneOfTheseNamespaces('App\Core\Component\**\Command', 'App\Core\Component\**\Event'))
+    ->should(new IsMapped($map))
+    ->because('we want to ensure our serializer can serialize/deserialize all commands and events');
+```
+
 ### Depends on a namespace
 
 ```php
diff --git a/src/Expression/ForClasses/IsMapped.php b/src/Expression/ForClasses/IsMapped.php
new file mode 100644
index 00000000..70cd0f77
--- /dev/null
+++ b/src/Expression/ForClasses/IsMapped.php
@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Expression\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Expression\Description;
+use Arkitect\Expression\Expression;
+use Arkitect\Rules\Violation;
+use Arkitect\Rules\ViolationMessage;
+use Arkitect\Rules\Violations;
+
+final class IsMapped implements Expression
+{
+    public const POSITIVE_DESCRIPTION = 'should exist in the list';
+
+    /** @var array */
+    private $list;
+
+    public function __construct(array $list)
+    {
+        $this->list = array_flip($list);
+    }
+
+    public function describe(ClassDescription $theClass, string $because): Description
+    {
+        return new Description(self::POSITIVE_DESCRIPTION, $because);
+    }
+
+    public function evaluate(ClassDescription $theClass, Violations $violations, string $because): void
+    {
+        if (isset($this->list[$theClass->getFQCN()])) {
+            return;
+        }
+
+        $violation = Violation::create(
+            $theClass->getFQCN(),
+            ViolationMessage::selfExplanatory($this->describe($theClass, $because))
+        );
+
+        $violations->add($violation);
+    }
+}
diff --git a/tests/Unit/Expressions/ForClasses/IsMappedTest.php b/tests/Unit/Expressions/ForClasses/IsMappedTest.php
new file mode 100644
index 00000000..3479dc28
--- /dev/null
+++ b/tests/Unit/Expressions/ForClasses/IsMappedTest.php
@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses;
+
+use Arkitect\Analyzer\ClassDescriptionBuilder;
+use Arkitect\Expression\ForClasses\IsMapped;
+use Arkitect\Rules\Violations;
+use PHPUnit\Framework\TestCase;
+
+final class IsMappedTest extends TestCase
+{
+    public function test_it_should_not_add_violation_if_fqcn_is_in_list(): void
+    {
+        $listedFqcn = 'App\SharedKernel\Component\MyClass';
+        $list = [
+            'a' => $listedFqcn,
+            'b' => 'App\SharedKernel\Component\MyOtherClass',
+        ];
+        $expression = new IsMapped($list);
+        $classDescription = (new ClassDescriptionBuilder())->setClassName($listedFqcn)->build();
+
+        $violations = new Violations();
+        $expression->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(0, $violations->count());
+    }
+
+    public function test_it_should_add_violation_if_fqcn_is_not_in_list(): void
+    {
+        $nonListedFqcn = 'App\SharedKernel\Component\MyClass';
+        $list = [
+            'a' => 'App\SharedKernel\Component\SomeClass',
+            'b' => 'App\SharedKernel\Component\MyOtherClass',
+        ];
+        $expression = new IsMapped($list);
+        $classDescription = (new ClassDescriptionBuilder())->setClassName($nonListedFqcn)->build();
+
+        $violations = new Violations();
+        $expression->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(1, $violations->count());
+        self::assertEquals(IsMapped::POSITIVE_DESCRIPTION, $expression->describe($classDescription, '')->toString());
+    }
+}