diff --git a/src/effect/EffectSchemaTraitExpressionBuilder.ts b/src/effect/EffectSchemaTraitExpressionBuilder.ts
index afa83db..42ad51e 100644
--- a/src/effect/EffectSchemaTraitExpressionBuilder.ts
+++ b/src/effect/EffectSchemaTraitExpressionBuilder.ts
@@ -1,5 +1,5 @@
 import { Schema as S } from "@effect/schema"
-import { Simplify } from "effect/Types"
+import { Mutable, Simplify } from "effect/Types"
 import { Trait, TraitTuple } from "../Trait"
 import { TraitExpression } from "../TraitExpression"
 import { TraitExpressionBuilder } from "../TraitExpressionBuilder"
@@ -92,6 +92,37 @@ export class EffectSchemaTraitExpressionBuilder<
     ) {}
 
 
+    mutable(): EffectSchemaTraitExpressionBuilder<
+        Fields,
+        Mutable<A>,
+        I,
+        R,
+        C,
+        Inherited,
+        Proto,
+        Static,
+
+        Traits
+    > {
+        return new EffectSchemaTraitExpressionBuilder(this.expressionSuperclass, this.expressionTraits)
+    }
+
+    mutableEncoded(): EffectSchemaTraitExpressionBuilder<
+        Fields,
+        A,
+        Mutable<I>,
+        R,
+        C,
+        Inherited,
+        Proto,
+        Static,
+
+        Traits
+    > {
+        return new EffectSchemaTraitExpressionBuilder(this.expressionSuperclass, this.expressionTraits)
+    }
+
+
     expresses<
         const T extends readonly Trait<
             TraitExpression<
diff --git a/src/effect/tests.ts b/src/effect/tests.ts
index 8910c9c..34b0b97 100644
--- a/src/effect/tests.ts
+++ b/src/effect/tests.ts
@@ -8,6 +8,8 @@ const userExp = effectSchemaExpression
         id:    S.BigIntFromSelf,
         role:  S.Union(S.Literal("User"), S.Literal("Admin")),
     })
+    .mutable()
+    .mutableEncoded()
     .build()
 
 @userExp.staticImplements
@@ -16,6 +18,10 @@ class User extends userExp.extends<User>() implements Implements<typeof userExp>
     static aStaticMethodThatShouldBeInherited() {}
 }
 
+const user = new User({ id: 0n, role: "User" })
+user.id = 0n
+type UserEncoded = S.Schema.Encoded<typeof User>
+
 
 const adminExp = effectSchemaExpression
     .extends(User, "User", {
@@ -27,4 +33,5 @@ const adminExp = effectSchemaExpression
 class Admin extends adminExp.extends<Admin>() implements Implements<typeof adminExp> {
 }
 
-new Admin({ id: 0n, role: "Admin" }).aMethodThatShouldBeInherited()
+const admin = new Admin({ id: 1n, role: "Admin" })
+// admin.id = 1n