diff --git a/workflow/scripts/filter_mpile.py b/workflow/scripts/filter_mpile.py
index 3a92656..660f76d 100755
--- a/workflow/scripts/filter_mpile.py
+++ b/workflow/scripts/filter_mpile.py
@@ -1,18 +1,34 @@
 #!/usr/bin/python3
 
-# Author: Tom van der Valk
+# Author: Tom van der Valk (modified by Diana Robledo-Ruiz and Verena Kutschera)
+
+# Filters a mpileup file for further processing in the GERP pipeline, produced with 
+# `samtools mpileup -aa -r contigname --no-output-ends sample.bam`
 
 import sys
-import operator
 
 for line in sys.stdin:
     splitted = line.strip().split("\t")
+
+    # Check if at least one sample is present in the mpileup
+    if len(splitted) < 5:
+        print("Error: Invalid input line:", line)
+        sys.stderr.write("Error: Invalid input line:" + line + '\n')
+        break  # Stop the loop if invalid line
+
+    # Process column 5 containing the mapped read bases
     nucleotide = splitted[4].upper()
     A,C,T,G = nucleotide.count("A"), nucleotide.count("C"), nucleotide.count("T"), nucleotide.count("G")
+    
+    # Print the nucleotide if it is A, C, T, or G, and if only one read mapped at the position
     if A+C+T+G == 1 and len(nucleotide) == 1:
-        print (nucleotide)
+        print(nucleotide)
+
+    # Remove any positions other than one of the four bases, e.g. deletions indicated by "*"
     elif A+C+T+G == 0:
-        print ("N")
+        print("N")
+
+    # Remove any other read bases, including those followed by indels (indicated by "+" and an integer and sequence, or "-" and an integer)
     else:
-        print ("N")
+        print("N")