diff --git a/src/tables/gsub.js b/src/tables/gsub.js
index 3fb27276..ea38ed6b 100644
--- a/src/tables/gsub.js
+++ b/src/tables/gsub.js
@@ -402,6 +402,21 @@ subtableMakers[6] = function makeLookup6(subtable) {
     check.assert(false, 'lookup type 6 format must be 1, 2 or 3.');
 };
 
+subtableMakers[7] = function makeLookup7(extensionSubstitutionTable) {
+    if (extensionSubstitutionTable.substFormat === 1) {
+        const extensionTable = subtableMakers[extensionSubstitutionTable.lookupType](extensionSubstitutionTable.extension);
+        const returnTable = new table.Table('extensionSubstitutionTable', [
+            {name: 'substFormat', type: 'USHORT', value: extensionSubstitutionTable.substFormat},
+            {name: 'extensionLookupType', type: 'USHORT', value: extensionSubstitutionTable.lookupType},
+            {name: 'extensionOffset', type: 'ULONG', value: undefined}
+        ]);
+        returnTable.extensionTable = extensionTable;
+        return returnTable;
+    }
+
+    check.assert(false, 'lookup type 7 format must be 1.');
+};
+
 function makeGsubTable(gsub) {
     return new table.Table('GSUB', [
         {name: 'version', type: 'ULONG', value: 0x10000},
diff --git a/src/types.js b/src/types.js
index 25898970..410d0443 100644
--- a/src/types.js
+++ b/src/types.js
@@ -945,6 +945,8 @@ encode.TABLE = function(table) {
     const length = (table.fields || []).length;
     const subtables = [];
     const subtableOffsets = [];
+    const extensionTables = [];
+    const extensionOffsets = [];
 
     for (let i = 0; i < length; i += 1) {
         const field = table.fields[i];
@@ -966,6 +968,11 @@ encode.TABLE = function(table) {
                 subtables.push(bytes);
             }
             d.push(...[0, 0]);
+            subtables.push(bytes);
+
+            if ( value.extensionTable ) {
+                extensionTables.push(encode.TABLE(value.extensionTable));
+            }
         } else {
             for (let j = 0; j < bytes.length; j++) {
                 d.push(bytes[j]);
@@ -976,7 +983,8 @@ encode.TABLE = function(table) {
     for (let i = 0; i < subtables.length; i += 1) {
         const o = subtableOffsets[i];
         const offset = d.length;
-        check.argument(offset < 65536, 'Table ' + table.tableName + ' too big.');
+        const maxOffset = table.tableName === 'lookupListTable' ? 4294967295 : 65536;
+        check.argument(offset < maxOffset, 'Table ' + table.tableName + ' too big.');
         d[o] = offset >> 8;
         d[o + 1] = offset & 0xff;
         for (let j = 0; j < subtables[i].length; j++) {
@@ -984,6 +992,12 @@ encode.TABLE = function(table) {
         }
     }
 
+    for (let i = 0; i < extensionTables.length; i += 1) {
+        // TODO: calculate offset value and replace in corresponding extensionSubstitutionTable
+        // Is this even the right spot to append the extensions data?
+        d = d.concat(extensionTables[i]);
+    }
+
     return d;
 };