diff --git a/singlecell/resources/chunks/AppendNimble.R b/singlecell/resources/chunks/AppendNimble.R
index 97023dea0..d24ac73bf 100644
--- a/singlecell/resources/chunks/AppendNimble.R
+++ b/singlecell/resources/chunks/AppendNimble.R
@@ -19,7 +19,8 @@ for (datasetId in names(seuratObjects)) {
   for (genomeId in names(nimbleGenomes)) {
     maxAmbiguityAllowed <- nimbleGenomeAmbiguousPreference[[genomeId]]
     queryDatabaseForLineageUpdates <- queryDatabaseForLineageUpdatesPreference[[genomeId]]
-    seuratObj <- Rdiscvr::DownloadAndAppendNimble(seuratObject = seuratObj, allowableGenomes = genomeId, ensureSamplesShareAllGenomes = ensureSamplesShareAllGenomes, targetAssayName = nimbleGenomes[[genomeId]], enforceUniqueFeatureNames = TRUE, maxAmbiguityAllowed = maxAmbiguityAllowed, maxLibrarySizeRatio = maxLibrarySizeRatio, queryDatabaseForLineageUpdates = queryDatabaseForLineageUpdates)
+    replaceExistingAssayData <- replaceExistingAssayDataByGenome[[genomeId]]
+    seuratObj <- Rdiscvr::DownloadAndAppendNimble(seuratObject = seuratObj, allowableGenomes = genomeId, ensureSamplesShareAllGenomes = ensureSamplesShareAllGenomes, targetAssayName = nimbleGenomes[[genomeId]], enforceUniqueFeatureNames = TRUE, maxAmbiguityAllowed = maxAmbiguityAllowed, maxLibrarySizeRatio = maxLibrarySizeRatio, queryDatabaseForLineageUpdates = queryDatabaseForLineageUpdates, replaceExistingAssayData = replaceExistingAssayData)
   }
 
   saveData(seuratObj, datasetId)
diff --git a/singlecell/resources/web/singlecell/panel/NimbleAppendPanel.js b/singlecell/resources/web/singlecell/panel/NimbleAppendPanel.js
index 1f748636d..9f346ff14 100644
--- a/singlecell/resources/web/singlecell/panel/NimbleAppendPanel.js
+++ b/singlecell/resources/web/singlecell/panel/NimbleAppendPanel.js
@@ -10,7 +10,7 @@ Ext4.define('SingleCell.panel.NimbleAppendPanel', {
 	initComponent: function(){
 		Ext4.apply(this, {
 			style: 'padding: 10px;margins: 5px;',
-            minWidth: 950,
+            minWidth: 1025,
 			border: true,
 			items: [{
 				html: 'This step will query nimble results for the selected genome(s). It will then append these results to the seurat object on the target assay.',
@@ -20,7 +20,7 @@ Ext4.define('SingleCell.panel.NimbleAppendPanel', {
 			},{
 				xtype: 'ldk-gridpanel',
 				clicksToEdit: 1,
-				width: 875,
+				width: 1000,
 				tbar: [{
 					text: 'Add',
 					handler: function(btn){
@@ -40,7 +40,7 @@ Ext4.define('SingleCell.panel.NimbleAppendPanel', {
 				},LABKEY.ext4.GRIDBUTTONS.DELETERECORD()],
 				store: {
 					type: 'array',
-					fields: ['genomeId', 'targetAssay','maxAmbiguityAllowed', 'queryDatabaseForLineageUpdates']
+					fields: ['genomeId', 'targetAssay','maxAmbiguityAllowed', 'queryDatabaseForLineageUpdates', 'replaceExistingAssayData']
 				},
 				columns: [{
 					dataIndex: 'genomeId',
@@ -86,6 +86,16 @@ Ext4.define('SingleCell.panel.NimbleAppendPanel', {
 						allowBlank: true,
 						value: false
 					}
+				},{
+					dataIndex: 'replaceExistingAssayData',
+					width: 150,
+					header: 'Replace Existing Data?',
+					editor: {
+						xtype: 'checkbox',
+						allowBlank: true,
+						value: true
+					}
+
 				}]
 			}]
 		});
@@ -96,7 +106,7 @@ Ext4.define('SingleCell.panel.NimbleAppendPanel', {
 	getValue: function(){
 		var ret = [];
 		this.down('ldk-gridpanel').store.each(function(r, i) {
-			ret.push([r.data.genomeId, r.data.targetAssay, r.data.maxAmbiguityAllowed ?? '', !!r.data.queryDatabaseForLineageUpdates]);
+			ret.push([r.data.genomeId, r.data.targetAssay, r.data.maxAmbiguityAllowed ?? '', !!r.data.queryDatabaseForLineageUpdates], !!r.data.replaceExistingAssayData);
 		}, this);
 
 		return Ext4.isEmpty(ret) ? null : JSON.stringify(ret);
@@ -133,7 +143,8 @@ Ext4.define('SingleCell.panel.NimbleAppendPanel', {
 					genomeId: row[0],
 					targetAssay: row[1],
 					maxAmbiguityAllowed: row[2],
-					queryDatabaseForLineageUpdates: !!row[3]
+					queryDatabaseForLineageUpdates: !!row[3],
+					replaceExistingAssayData: !!row[4]
 				});
 				grid.store.add(rec);
 			}, this);
diff --git a/singlecell/src/org/labkey/singlecell/pipeline/singlecell/AppendNimble.java b/singlecell/src/org/labkey/singlecell/pipeline/singlecell/AppendNimble.java
index 087abeac6..093c56e44 100644
--- a/singlecell/src/org/labkey/singlecell/pipeline/singlecell/AppendNimble.java
+++ b/singlecell/src/org/labkey/singlecell/pipeline/singlecell/AppendNimble.java
@@ -77,7 +77,7 @@ protected Chunk createParamChunk(SequenceOutputHandler.JobContext ctx, List<Seur
         for (int i = 0; i < json.length(); i++)
         {
             JSONArray arr = json.getJSONArray(i);
-            if (arr.length() != 4)
+            if (arr.length() != 5)
             {
                 throw new PipelineJobException("Unexpected value: " + json.get(i));
             }
@@ -96,7 +96,7 @@ protected Chunk createParamChunk(SequenceOutputHandler.JobContext ctx, List<Seur
         for (int i = 0; i < json.length(); i++)
         {
             JSONArray arr = json.getJSONArray(i);
-            if (arr.length() < 3)
+            if (arr.length() != 5)
             {
                 throw new PipelineJobException("Unexpected value: " + json.getString(i));
             }
@@ -118,13 +118,32 @@ protected Chunk createParamChunk(SequenceOutputHandler.JobContext ctx, List<Seur
         for (int i = 0; i < json.length(); i++)
         {
             JSONArray arr = json.getJSONArray(i);
-            if (arr.length() != 4)
+            if (arr.length() != 5)
             {
                 throw new PipelineJobException("Unexpected value: " + json.getString(i));
             }
 
             int genomeId = arr.getInt(0);
-            String valStr = arr.get(2) == null ? null : StringUtils.trimToNull(String.valueOf(arr.get(2)));
+            String valStr = arr.get(3) == null ? null : StringUtils.trimToNull(String.valueOf(arr.get(2)));
+            boolean val = Boolean.parseBoolean(valStr);
+
+            ret.bodyLines.add("\t" + delim + "'" + genomeId + "' = " + (val ? "TRUE" : "FALSE"));
+            delim = ",";
+        }
+        ret.bodyLines.add(")");
+
+        ret.bodyLines.add("replaceExistingAssayDataByGenome <- list(");
+        delim = "";
+        for (int i = 0; i < json.length(); i++)
+        {
+            JSONArray arr = json.getJSONArray(i);
+            if (arr.length() != 5)
+            {
+                throw new PipelineJobException("Unexpected value: " + json.getString(i));
+            }
+
+            int genomeId = arr.getInt(0);
+            String valStr = arr.get(4) == null ? null : StringUtils.trimToNull(String.valueOf(arr.get(2)));
             boolean val = Boolean.parseBoolean(valStr);
 
             ret.bodyLines.add("\t" + delim + "'" + genomeId + "' = " + (val ? "TRUE" : "FALSE"));