diff --git a/src/main/java/com/ankurm/jackson3/part4custom/E04MixinAnnotations.java b/src/main/java/com/ankurm/jackson3/part4custom/E04MixinAnnotations.java
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+++ b/src/main/java/com/ankurm/jackson3/part4custom/E04MixinAnnotations.java
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+package com.ankurm.jackson3.part4custom;
+
+import com.fasterxml.jackson.annotation.JsonIgnore;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import tools.jackson.databind.json.JsonMapper;
+
+/**
+ * Post: Custom Serialisers and Mix-ins — https://ankurm.com/jackson-custom-serializer-mixin/
+ * Section: "Mix-in Annotations — Annotating Third-Party Classes"
+ *
+ * Jackson 3 difference: mixins are registered on the BUILDER (addMixIn), because
+ * mapper.addMixIn(...) does not exist on an immutable mapper.
+ */
+public class E04MixinAnnotations {
+
+    /** Stand-in for a third-party class whose source you cannot modify. */
+    public static class Address {
+        public String street;
+        public String city;
+        public String postalCode;
+        public String internalTrackingCode;   // must never reach the wire
+    }
+
+    /** Mix-in: carries the annotations Jackson should apply to Address. */
+    public abstract static class AddressMixin {
+        @JsonIgnore            public String internalTrackingCode;  // suppress
+        @JsonProperty("zip")   public String postalCode;            // rename
+    }
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder()
+                .addMixIn(Address.class, AddressMixin.class)
+                .build();
+
+        Address address = new Address();
+        address.street               = "123 Main St";
+        address.city                 = "Springfield";
+        address.postalCode           = "12345";
+        address.internalTrackingCode = "INTERNAL-X99";
+
+        System.out.println("with mixin   : " + mapper.writeValueAsString(address));
+
+        // The target class is untouched — a mapper without the mixin still sees
+        // every field under its original name.
+        System.out.println("without mixin: "
+                + JsonMapper.builder().build().writeValueAsString(address));
+    }
+}