diff --git a/refinegems/gapfill.py b/refinegems/gapfill.py
index 0fb368b9..fd069458 100644
--- a/refinegems/gapfill.py
+++ b/refinegems/gapfill.py
@@ -1,24 +1,82 @@
-# Function originally from refineGEMs.genecomp/refineGEMs.KEGG_analysis --- Modified
-def compare_gene_lists(gps_in_model: pd.DataFrame, db_genes: pd.DataFrame) -> pd.DataFrame:
-   
-   in_db = db_genes.set_index('Locus_tag')
-   in_model = gps_in_model.set_index(0)
-   genes_in_db_not_in_model = in_db[~in_db.index.isin(in_model.index)]
-   return genes_in_db_not_in_model.reset_index()
+#!/usr/bin/env python
+from libsbml import *
+import refinegems.analysis_db as rga
+import refinegems.analysis_kegg as rga_kegg
+import refinegems.analysis_biocyc as rga_biocyc
+import refinegems.entities as rge
+from colorama import init as colorama_init
+from colorama import Fore
 
-def get_genes_from_gff():
-   pass
+__author__ = "Famke Baeuerle and Gwendolyn O. Gusak"
 
 
-def compare_model_to_gff():
-   pass
+def get_genes_from_gff():
+    pass
 
 
 def get_related_metabs_reactions_blast():
-   pass
+    pass
+
 
+def gff_gene_comp():
+    pass
 
-def gapfill():
-   ''' Main function to gapfill a model with comparison to KEGG/BioCyc/(Genbank) GFF file
-   '''
-   pass
+
+def gapfill_analysis(model_libsbml: Model, gapfill_params: dict[str: str]):
+    """ Main function to gapfill a model with comparison to KEGG/BioCyc/(Genbank) GFF file
+   
+        Args:
+            model_libsbml (Model): model loaded with libSBML
+            gapfill_params (dict): Dictionary obtained from YAML file containing the parameter mappings
+    """
+    colorama_init(autoreset=True)
+    db_to_compare = gapfill_params['db_to_compare']
+    
+    if db_to_compare not in ['KEGG', 'BioCyc', 'GFF', 'KEGG+BioCyc']:
+        print(f'{Fore.RED}To use the module gapfill the parameter of db_to_compare has to be set to one of the following' 
+              + ' options:\n- \'KEGG\'\n- \'BioCyc\'\n- \'GFF\'\n- \'KEGG+BioCyc\'\nAdditionally, the required parameters' 
+              + ' for each option need to be specified.\n- \'biggreactions\' and \'gapfill\' are required for all options.'
+              + '\n- \'organismid\' is required only for the options \'KEGG\' and \'KEGG+BioCyc\'.\n- \'biocyc_tables\'' 
+              + ' is only required for the options \'BioCyc\' and \'KEGG+BioCyc\'.')
+        return
+   
+    if db_to_compare == 'KEGG':
+        if gapfill_params['organismid']:
+            missing_kegg = rga_kegg.kegg_gene_comp(model_libsbml, 
+                                                   gapfill_params['organismid'], 
+                                                   gapfill_params['bigg_dbs'][0], 
+                                                   gapfill_params['gff_file']
+                                                   )
+            return missing_kegg
+        else:
+            print(f'{Fore.RED}To use the KEGG comparison the specification of the organismid is obligatory.\n' +
+                  'If there is no organismid available for your organism in KEGG, use one of the options \'BioCyc\' or \'GFF\'.')
+        
+    elif db_to_compare == 'BioCyc':
+        missing_biocyc = rga_biocyc.biocyc_gene_comp(model_libsbml, 
+                                                     gapfill_params['biocyc_files'],
+                                                     gapfill_params['bigg_dbs']
+                                                     )
+        return missing_biocyc
+        
+    elif db_to_compare == 'GFF':
+        gff_genes = gff_gene_comp(model_libsbml, 
+                                  gapfill_params['bigg_dbs'], 
+                                  gapfill_params['gff_file']
+                                  )
+        return gff_genes
+        
+    elif db_to_compare == 'KEGG+BioCyc':
+        missing_kegg_reacs = rga_kegg.kegg_gene_comp(model_libsbml, 
+                                               gapfill_params['organismid'], 
+                                               gapfill_params['bigg_dbs'][0], 
+                                               gapfill_params['gff_file']
+                                               )
+        missing_kegg_reacs.drop(['name', 'locus_tag', 'EC'], axis=1, inplace=True)
+        missing_biocyc = rga_biocyc.biocyc_gene_comp(model_libsbml, 
+                                                     gapfill_params['biocyc_files'],
+                                                     gapfill_params['bigg_dbs']
+                                                     )
+        stats, missing_biocyc_genes, missing_biocyc_metabs, missing_metabs_wo_BiGG_df, missing_biocyc_reacs = missing_biocyc
+        missing_combined_reacs = missing_biocyc_reacs.merge(missing_kegg_reacs, how='left', on='bigg_id')
+        return (stats, missing_biocyc_genes, missing_biocyc_metabs, missing_metabs_wo_BiGG_df, missing_combined_reacs, missing_kegg_reacs)