diff --git a/modelling/pymod/_afdb_modelling.py b/modelling/pymod/_afdb_modelling.py
index ae12b4db66cef1cf10efae230542ff919ebdbe43..08073ee49a67511b53aeff8b61cf705df1fb91e9 100644
--- a/modelling/pymod/_afdb_modelling.py
+++ b/modelling/pymod/_afdb_modelling.py
@@ -168,8 +168,9 @@ class FSStructureServer:
             tmp.sort()
             sorted_data_files = [x[1] for x in tmp]
             for f in sorted_data_files:
-                self._data_fh.append(open(f, 'r+b'))
-                self._data.append(mmap.mmap(self._data_fh[-1].fileno(), 0))
+                self._data_fh.append(open(f, 'rb'))
+                self._data.append(mmap.mmap(self._data_fh[-1].fileno(), 0,
+                                            prot=mmap.PROT_READ))
         return self._data
 
     @property