diff --git a/packages/nightingale-structure/src/position-mapping.ts b/packages/nightingale-structure/src/position-mapping.ts
index 0bcf7ba9..d5d75bd1 100644
--- a/packages/nightingale-structure/src/position-mapping.ts
+++ b/packages/nightingale-structure/src/position-mapping.ts
@@ -1,5 +1,3 @@
-import groupBy from "lodash-es/groupBy";
-
 export type Direction = "UP_PDB" | "PDB_UP";
 
 /* eslint-disable camelcase */
@@ -34,65 +32,76 @@ export type TranslatedPosition = {
 /**
  * Translate between UniProt and PDBe positions using SIFTs mappings
  * @function translatePositions
- * @param  {Number}     start            The start index for the sequence (1-based)
- * @param  {Number}     end              The end index for the sequence (1-based)
- * @param  {Mapping[]}   mappings         The array of mapping objects
- * @param  {String}     mappingDirection Indicates direction of maping: UniProt to PDB or PDB to UniProt
- * @return {Translated}                  Object with: mapped entity ID; mapped chain ID; translated start & end positions
+ * @param  {number}     start            The start index for the sequence (1-based)
+ * @param  {number}     end              The end index for the sequence (1-based)
+ * @param  {Direction}  mappingDirection Indicates direction of mapping: UniProt to PDB or PDB to UniProt
+ * @param  {Mapping[]}  mappings         The array of mapping objects
+ * @return {TranslatedPosition[]}        Array of translated positions
  */
 const translatePositions = (
   start: number,
   end: number,
   mappingDirection: Direction,
-  mappings?: Mapping[],
+  mappings?: Mapping[]
 ): TranslatedPosition[] => {
   // Return if mappings not ready
   if (!mappings) {
     return [];
   }
-  // return if they have been set to 'undefined'
+
+  // Validate start and end inputs
   if (!start || !end || Number.isNaN(start) || Number.isNaN(end)) {
     throw new PositionMappingError("Invalid start, end coordinates");
   }
-  // return a translation separately for each chain (if it exists)
-  const translations: TranslatedPosition[] = Object.values(
-    groupBy(mappings, (mapping) => mapping.chain_id),
-  )
-    .map((chainMappings) => {
-      let startMapping = null;
-      let endMapping = null;
-      for (const mapping of chainMappings) {
-        if (
-          mapping.unp_end - mapping.unp_start !==
-          mapping.end.residue_number - mapping.start.residue_number
-        ) {
-          throw new PositionMappingError(
-            "Mismatch between protein sequence and structure residues",
-          );
-        }
-        const regionStart =
-          mappingDirection === "UP_PDB"
-            ? mapping.unp_start
-            : mapping.start.residue_number;
-        const regionEnd =
-          mappingDirection === "UP_PDB"
-            ? mapping.unp_end
-            : mapping.end.residue_number;
-
-        if (start >= regionStart && start <= regionEnd) {
-          startMapping = mapping;
-        }
-        if (end >= regionStart && end <= regionEnd) {
-          endMapping = mapping;
-        }
+
+  const chainMappingsMap = new Map<string, Mapping[]>();
+
+  // Group mappings by chain_id
+  for (const mapping of mappings) {
+    const chainMapping = chainMappingsMap.get(mapping.chain_id) || [];
+    chainMapping.push(mapping);
+    chainMappingsMap.set(mapping.chain_id, chainMapping);
+  }
+
+  const translations: TranslatedPosition[] = [];
+
+  for (const chainMappings of chainMappingsMap.values()) {
+    let startMapping: Mapping | null = null;
+    let endMapping: Mapping | null = null;
+
+    for (const mapping of chainMappings) {
+      if (
+        mapping.unp_end - mapping.unp_start !==
+        mapping.end.residue_number - mapping.start.residue_number
+      ) {
+        throw new PositionMappingError(
+          "Mismatch between protein sequence and structure residues"
+        );
       }
-      if (startMapping === null || endMapping === null) {
-        // If we didn't find a mapping for this chain,
-        // return null, it will be filtered out later
-        return null;
+
+      const [regionStart, regionEnd] =
+        mappingDirection === "UP_PDB"
+          ? [mapping.unp_start, mapping.unp_end]
+          : [mapping.start.residue_number, mapping.end.residue_number];
+
+      // Set startMapping if the start coordinate is within the region
+      if (!startMapping && start >= regionStart && start <= regionEnd) {
+        startMapping = mapping;
+      }
+      // Set endMapping if the end coordinate is within the region
+      if (!endMapping && end >= regionStart && end <= regionEnd) {
+        endMapping = mapping;
+      }
+
+      // Early exit if both start and end mappings are found
+      if (startMapping && endMapping) {
+        break;
       }
+    }
+
+    if (startMapping && endMapping) {
       const direction = mappingDirection === "UP_PDB" ? 1 : -1;
-      return {
+      translations.push({
         entity: startMapping.entity_id,
         // Note that we iterate through mappings from one chain at a time,
         // so all mappings are guaranteed to come from the same chain
@@ -104,15 +113,13 @@ const translatePositions = (
         end:
           end +
           direction * (endMapping.start.residue_number - endMapping.unp_start),
-      };
-    })
-    .filter((tp: TranslatedPosition | null): tp is TranslatedPosition =>
-      Boolean(tp),
-    );
+      });
+    }
+  }
 
   if (!translations.length) {
     throw new PositionMappingError(
-      "Start or end coordinate outside of mapping range",
+      "Start or end coordinate outside of mapping range"
     );
   }