diff --git a/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/PositionListWritable.java b/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/PositionListWritable.java
index f135292..b056c14 100644
--- a/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/PositionListWritable.java
+++ b/genomix/genomix-data/src/main/java/edu/uci/ics/genomix/type/PositionListWritable.java
@@ -4,6 +4,7 @@
 import java.io.DataOutput;
 import java.io.IOException;
 import java.io.Serializable;
+import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
 
@@ -76,6 +77,27 @@
         }
     }
     
+    /**
+     * Save the union of my list and otherList. Uses a temporary HashSet for
+     * uniquefication
+     */
+    public void unionUpdate(PositionListWritable otherList) {
+        int newSize = valueCount + otherList.valueCount;
+        HashSet<PositionWritable> uniqueElements = new HashSet<PositionWritable>(
+                newSize);
+        for (PositionWritable pos : this) {
+            uniqueElements.add(pos);
+        }
+        for (PositionWritable pos : otherList) {
+            uniqueElements.add(pos);
+        }
+        valueCount = 0;
+        setSize(newSize);
+        for (PositionWritable pos : uniqueElements) {
+            append(pos);
+        }
+    }
+    
     public static int getCountByDataLength(int length) {
         if (length % PositionWritable.LENGTH != 0) {
             throw new IllegalArgumentException("Length of positionlist is invalid");