diff --git a/src/pp5/prec.py b/src/pp5/prec.py
index 2af8f8a..f56e410 100644
--- a/src/pp5/prec.py
+++ b/src/pp5/prec.py
@@ -87,8 +87,8 @@
     backbone_only=True, unit_cell=None, isotropic=True, scale_as_bfactor=True
 )
 
-# Special insertion code to mark a residue that was missing from the structure.
-ICODE_MISSING_RESIDUE = "M"
+# Special insertion code to mark a residue that is unmodeled the structure.
+ICODE_UNMODELED_RES = "UNMODELED"
 
 
 class ResidueRecord(object):
@@ -899,7 +899,7 @@ def __init__(
                 missing_res_name_single = pdb_meta_aa_seq[curr_meta_seq_idx]
                 missing_res_name = ACIDS_1TO3[missing_res_name_single]
                 curr_residue = Residue(
-                    (" ", curr_meta_seq_idx, ICODE_MISSING_RESIDUE), missing_res_name, 0
+                    (" ", curr_meta_seq_idx, ICODE_UNMODELED_RES), missing_res_name, 0
                 )
                 missing_residues.append(curr_residue)