diff --git a/src/to_json.cpp b/src/to_json.cpp
index eb76ee6a..73cad8d3 100644
--- a/src/to_json.cpp
+++ b/src/to_json.cpp
@@ -128,12 +128,25 @@ class JsonWriter {
     else if (opt.quote_numbers < 2 && is_numb(value) &&
              // exception: 012 (but not 0.12) is assumed to be a string
              (value[0] != '0' || value[1] == '.' || value[1] == '\0') &&
+             // exception: digit-string + E + digit-string (e.g. 6E72) is
+             // assumed to be an identifier such as a PDB code, not a number
+             !looks_like_numberlike_id(value) &&
              (opt.quote_numbers == 0 || value.back() != ')'))
       write_as_number(value);
     else
       write_string(as_string(value));
   }
 
+  // returns true for tokens of the form digits + E + digits (e.g. 6E72):
+  // valid CIF numbers but indistinguishable from alphanumeric identifiers
+  // such as PDB codes, which a JSON reader would parse as huge floats
+  static bool looks_like_numberlike_id(const std::string& v) {
+    size_t epos = v.find('E');
+    return epos != std::string::npos &&
+           v.find_first_not_of("0123456789") == epos &&
+           v.find_first_not_of("0123456789", epos + 1) == std::string::npos;
+  }
+
   void open_cat(const std::string& cat, size_t* tag_pos) {
     if (!cat.empty()) {
       change_indent(+1);
diff --git a/tests/test_cif_json.py b/tests/test_cif_json.py
index f3931008..5c92f5b2 100644
--- a/tests/test_cif_json.py
+++ b/tests/test_cif_json.py
@@ -52,6 +52,13 @@ def test_json_number_normalization(self):
         self.assertEqual(parsed, {'test': {'_a': -0.99, '_b': 0.04,
                                            '_c': -0.04, '_d': 0.5}})
 
+    def test_json_numberlike_id(self):
+        # 6E72 is a valid CIF number but also a PDB code - quote it (#444)
+        doc = cif.read_string('data_6E72 _entry.id 6E72 _a 1.2E3 _b 2e4')
+        parsed = json.loads(doc.as_json())
+        self.assertEqual(parsed, {'6e72': {'_entry.id': '6E72',
+                                           '_a': 1200.0, '_b': 20000.0}})
+
 class TestMmjson(unittest.TestCase):
     def test_read_1pfe(self):
         path = full_path('1pfe.json')