diff --git a/src/db_utils/frequency_scripts/config_freq.py b/src/db_utils/frequency_scripts/config_freq.py
index 4f70c3c4c8feb7663e9ee9b87e91210df02b1fdb..c30fdf5234c7ef681340331cffdeabc5056ea194 100644
--- a/src/db_utils/frequency_scripts/config_freq.py
+++ b/src/db_utils/frequency_scripts/config_freq.py
@@ -71,8 +71,8 @@ class Config_freq:
     output = Config.results / 'frequency_table'
     output_exon_file = output / 'exon_freq.txt'
     output_gene_file = output / 'gene_freq.txt'
-    iupac_dic = {"S": ["G", "C"], "W": ["A", "T"], "K": ["G", "T"],
-                 "M": ["A", "C"], "R": ["A", "G"], "Y": ["C", "T"]}
+    iupac_dic = {"S": "[GC]", "W": "[AT]", "K": "[GT]",
+                 "M": "[AC]", "R": "[AG]", "Y": "[CT]"}
     nt_list = compute_nt_list()
     aa_list = list("RHKDESTNQCGPAVILMFYW")
     codon_list = [nt for nt in nt_list if len(nt) == 3]