diff --git a/src/effect/schema/MutableClass.ts b/src/effect/schema/MutableClass.ts
index d2ee108..9eada34 100644
--- a/src/effect/schema/MutableClass.ts
+++ b/src/effect/schema/MutableClass.ts
@@ -6,11 +6,11 @@ import type { Static } from "../.."
 import type { HasFields, MissingSelfGeneric } from "./util"
 
 
-type TMutableClass<
+export type TMutableClass<
     Self,
-    Fields    extends Struct.Fields,
+    Fields extends Struct.Fields,
     I, R, C,
-    Inherited extends object,
+    Inherited,
     Proto,
 > = (
     Constructor<
@@ -22,13 +22,32 @@ type TMutableClass<
 
         ConstructorParameters<Class<Self, Fields, I, R, C, Inherited, Proto>>
     > &
-    Static<Class<Self, Fields, I, R, C, Inherited, Proto>>
+
+    Omit<
+        Static<Class<Self, Fields, I, R, C, Inherited, Proto>>,
+        "extend"
+    > & {
+        extend<Extended = never>(identifier: string): <newFields extends Struct.Fields>(
+            fields:        newFields | HasFields<newFields>,
+            annotations?:  Annotations.Schema<Extended>,
+        ) => [Extended] extends [never]
+            ? MissingSelfGeneric<"Base.extend">
+            : TMutableClass<
+                Extended,
+                Fields & newFields,
+                I & Struct.Encoded<newFields>,
+                R | Struct.Context<newFields>,
+                C & Struct.Constructor<newFields>,
+                Self,
+                Proto
+            >
+    }
 )
 
 export const MutableClass = <Self = never>(identifier: string) => (
     <Fields extends Struct.Fields>(
         fieldsOr:      Fields | HasFields<Fields>,
-        annotations?:  Annotations.Schema<Self>
+        annotations?:  Annotations.Schema<Self>,
     ): (
         [Self] extends [never]
             ? MissingSelfGeneric<"Class">
diff --git a/src/effect/schema/index.ts b/src/effect/schema/index.ts
index 8c87023..d6234ea 100644
--- a/src/effect/schema/index.ts
+++ b/src/effect/schema/index.ts
@@ -1 +1,2 @@
 export { MutableClass } from "./MutableClass"
+export type { TMutableClass } from "./MutableClass"
diff --git a/src/effect/schema/tests.ts b/src/effect/schema/tests.ts
index 8d1ce62..adc1818 100644
--- a/src/effect/schema/tests.ts
+++ b/src/effect/schema/tests.ts
@@ -3,8 +3,9 @@ import { MutableClass } from "./MutableClass"
 
 
 class User extends MutableClass<User>("User")({
-    id: S.BigIntFromSelf
+    id: S.BigIntFromSelf,
+    role: S.Union(S.Literal("BasicUser"), S.Literal("Admin")),
 }) {}
 
-const user = new User({ id: 1n })
-user.id = 2n
+const user1 = new User({ id: 1n, role: "BasicUser" })
+user1.id = 2n