diff --git a/src/mol-model/structure/model/properties/sequence.ts b/src/mol-model/structure/model/properties/sequence.ts
index 229c5509b72ccff1ec691a867bdfa3f1e1f44a1b..f10338b174124e571eb7f069bd0d34da39eb8001 100644
--- a/src/mol-model/structure/model/properties/sequence.ts
+++ b/src/mol-model/structure/model/properties/sequence.ts
@@ -12,6 +12,12 @@ interface Sequence {
     readonly byEntityKey: { [key: number]: Sequence.Entity }
 }
 
+// TODO lift to model/sequence/ folder
+// TODO add one letter code sequence string
+// TODO add mapping support to other sequence spaces, e.g. uniprot
+// TODO add sequence kind, e.g. protein, dna, rna (alphabets?)
+// TODO sequence alignment (take NGL code as starting point)
+
 namespace Sequence {
     export interface Entity {
         readonly entityId: string,
@@ -27,6 +33,9 @@ namespace Sequence {
 
         const byEntityKey: Sequence['byEntityKey'] = {};
 
+        // TODO get min/max of label_seq_id to handle missing residues at start and in between
+        //   note that this assumes label_seq_id is monotonically increasing
+
         const chainCount = hierarchy.chains._rowCount
         for (let i = 0; i < chainCount; ++i) {
             const entityId = label_entity_id.value(i)
diff --git a/src/mol-model/structure/structure/element.ts b/src/mol-model/structure/structure/element.ts
index e62dcf1333a3585005a1075391a2314152e4f21d..48e0307605f4516c66b8540c46f74fefb05340a2 100644
--- a/src/mol-model/structure/structure/element.ts
+++ b/src/mol-model/structure/structure/element.ts
@@ -4,7 +4,7 @@
  * @author David Sehnal <david.sehnal@gmail.com>
  */
 
-import { Tuple } from 'mol-data/int'
+import { Tuple, SortedArray } from 'mol-data/int'
 import Unit from './unit'
 import Structure from './structure'
 
@@ -35,6 +35,9 @@ namespace Element {
     }
 
     export function property<T>(p: Property<T>) { return p; }
+
+    /** Represents multiple element locations */
+    export type Loci = ReadonlyArray<{ unit: Unit, elements: SortedArray }>
 }
 
 export default Element
\ No newline at end of file