diff --git a/modules/mmcif/pyext/src/data.py b/modules/mmcif/pyext/src/data.py
index 7cb40a70ed..53b8fbfac0 100644
--- a/modules/mmcif/pyext/src/data.py
+++ b/modules/mmcif/pyext/src/data.py
@@ -93,10 +93,30 @@ def __init__(self, system):
             IMP.atom.RNA: ihm.RNAAlphabet,
             IMP.atom.DNA: _CustomDNAAlphabet}
 
-    def add(self, chain):
+    def _get_sequence_from_residues(self, chain, seq_from_res):
+        seq_id_begin, seq = seq_from_res
+        # todo: handle seq_id_begin != 1
+        if not seq:
+            raise ValueError("Chain %s has no sequence and no residues"
+                             % chain)
+        missing_seq = [ind + seq_id_begin
+                       for (ind, res) in enumerate(seq) if res is None]
+        if missing_seq:
+            raise ValueError(
+                "Chain %s has no declared sequence; tried to determine the "
+                "sequence from Residues, but the following residue indices "
+                "have no residue type (perhaps covered only by Fragments): %s"
+                % (chain, str(missing_seq)))
+        return tuple(seq)
+
+    def add(self, chain, seq_from_res=None):
         sequence = chain.get_sequence()
         if sequence == '':
-            raise ValueError("Chain %s has no sequence" % chain)
+            if seq_from_res is not None:
+                sequence = self._get_sequence_from_residues(chain,
+                                                            seq_from_res)
+            else:
+                raise ValueError("Chain %s has no sequence" % chain)
         else:
             # Map one-letter codes to ihm.ChemComp
             alphabet = self._alphabet_map[chain.get_chain_type()]()
diff --git a/modules/mmcif/pyext/src/util.py b/modules/mmcif/pyext/src/util.py
index f84b0a1a42..51a3129ccf 100644
--- a/modules/mmcif/pyext/src/util.py
+++ b/modules/mmcif/pyext/src/util.py
@@ -269,7 +269,7 @@ def _add_state(self, state):
         self._state_by_name[state.name] = state
 
     def _add_chain(self, chain, seq_from_res):
-        entity = self._entities.add(chain)
+        entity = self._entities.add(chain, seq_from_res)
         component = self._components.add(chain, entity)
         return component
 
diff --git a/modules/mmcif/test/test_data.py b/modules/mmcif/test/test_data.py
index 8e74092e71..2104e85c73 100644
--- a/modules/mmcif/test/test_data.py
+++ b/modules/mmcif/test/test_data.py
@@ -81,6 +81,17 @@ def test_entity_mapper_add(self):
         # Cannot add chains with no sequence
         chain5 = MockChain("E", sequence='')
         self.assertRaises(ValueError, e.add, chain5)
+        # Chain with no declared sequence, but we have from-residue sequence
+        alpha = ihm.LPeptideAlphabet()
+        e.add(chain5, seq_from_res=(1, (alpha['C'], alpha['G'])))
+        # List should work as well as tuple
+        e.add(chain5, seq_from_res=(1, [alpha['C'], alpha['G']]))
+        # Will not work if from-residue sequence is also empty
+        self.assertRaises(ValueError, e.add, chain5, seq_from_res=(1, ()))
+        # Also no good if the from-residue sequence has gaps
+        self.assertRaises(ValueError, e.add, chain5,
+                          seq_from_res=(1, (alpha['C'], None,
+                                            alpha['G'], None)))
 
     def test_entity_naming(self):
         """Test naming of Entities"""