diff --git a/tests/test_core.py b/tests/test_core.py
index 9f81c83..c400e37 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -172,7 +172,8 @@ def test(self):
         self.tmp_dirname = tempfile.mkdtemp()
         filepath = os.path.join(self.tmp_dirname, 'test_seq.fasta')
         with open(filepath, 'w') as f:
-            f.write('>dna1\nACGTACGT\n')
+            f.write('>dna1\nACGTACGT\n'
+                    '>dna2\nACGTACGTNNNN\n')
 
         dna = core.DnaSpeciesType(id='dna1', name='dna1', sequence_path=filepath,
                                   circular=False, double_stranded=False, ploidy=2)
@@ -270,6 +271,19 @@ def test(self):
 
         self.assertAlmostEqual(dna.get_mol_wt(), exp_mol_wt, places=0)
 
+        # If there are N's in the DNA sequence
+        dna2 = core.DnaSpeciesType(id='dna2', sequence_path=filepath,
+                                  circular=False, double_stranded=True)
+
+        L = dna2.get_len()
+        self.assertEqual(dna2.get_empirical_formula(),
+                         chem.EmpiricalFormula('C10H12N5O6P') * 3 * 2
+                         + chem.EmpiricalFormula('C9H12N3O7P') * 3 * 2
+                         + chem.EmpiricalFormula('C10H12N5O7P') * 3 * 2
+                         + chem.EmpiricalFormula('C10H13N2O8P') * 3 * 2
+                         - chem.EmpiricalFormula('OH') * (L - 1) * 2
+                         )
+
         shutil.rmtree(self.tmp_dirname)  
 
 
diff --git a/wc_kb/core.py b/wc_kb/core.py
index 56070d5..a8bb904 100644
--- a/wc_kb/core.py
+++ b/wc_kb/core.py
@@ -1594,16 +1594,25 @@ def get_empirical_formula(self):
 
             :math:`N_A * dAMP + N_C * dCMP + N_G * dGMP + N_T * dTMP - L * OH`
 
+        N's in the sequence will be distributed into the four bases by preserving the original ratio    
+
         Returns:
            :obj:`chem.EmpiricalFormula`: empirical formula
         """
         seq = self.get_seq()
-        n_a = seq.count('A')
-        n_c = seq.count('C')
-        n_g = seq.count('G')
-        n_t = seq.count('T')
-        l = len(seq)
-
+        n_a = seq.upper().count('A')
+        n_c = seq.upper().count('C')
+        n_g = seq.upper().count('G')
+        n_t = seq.upper().count('T')
+        n_n = seq.upper().count('N')
+        
+        l = len(seq)         
+        known_bases = n_a + n_c + n_g + n_t
+        n_a += round(n_a / known_bases * n_n)
+        n_c += round(n_c / known_bases * n_n)
+        n_g += round(n_g / known_bases * n_n)        
+        n_t = l - (n_a + n_c + n_g)
+       
         if self.double_stranded:
             n_a = n_a + n_t
             n_t = n_a