diff --git a/src/mol-data/int/segmentation.ts b/src/mol-data/int/segmentation.ts
index 94abaaa0f1b823c16ff5b37e252e7d5b7c40af8e..111914201f1f8e620c9fe5ded7f4f10119045373 100644
--- a/src/mol-data/int/segmentation.ts
+++ b/src/mol-data/int/segmentation.ts
@@ -20,6 +20,8 @@ namespace Segmentation {
 
     // Segment iterator that mutates a single segment object to mark all the segments.
     export const transientSegments: <T extends number = number>(segs: Segmentation<T>, set: OrderedSet<T>, segment?: Segment<T>) => Impl.SegmentIterator<T> = Impl.segments as any;
+
+    export type Iterator<T extends number = number> = Impl.SegmentIterator<T>
 }
 
 interface Segmentation<T extends number = number> {