diff --git a/rnaseq/umi/07_remove_dups.pl b/rnaseq/umi/07_remove_dups.pl
index 2aa2859..8834e38 100644
--- a/rnaseq/umi/07_remove_dups.pl
+++ b/rnaseq/umi/07_remove_dups.pl
@@ -2,6 +2,7 @@
 use strict;
 use Parallel::ForkManager;
 # this script creats one file with UMI unique reads and one with UMI duplicated reads
+# as input you need aligned sorted by coordinate bam file
 my @torun = ();
 foreach my $file ( <*Aligned.sortedByCoord.out.bam> ) {
     push @torun, $file;
@@ -33,12 +34,12 @@ foreach my $file ( @torun ) {
         my $uniq = join( ':', $chr, $pos, $flag, $tlen, $bc );
         my $pos_ = $pos-1;
         while ( $cigar =~ m/(\d+)([SHMDIN=])/g ) {
-            $pos_+=$1 if $2 eq 'M' or $2 eq '=' or $2 eq 'D' or $2 eq 'N'; 
+            $pos_+=$1 if $2 eq 'M' or $2 eq '=' or $2 eq 'D' or $2 eq 'N'; # find position for minus strand
         }
         my $uniq2 = join( ':', $chr, $pos_, $flag, $tlen, $bc );
         if ( exists($duplicates{$id}) or # already marked as duplicate
             ( not($flag&16) and ++$seen{ $uniq } > 1 ) or # plus strand
-            ( $flag&16 and ++$seen{ $uniq2 } > 1 )
+            ( $flag&16 and ++$seen{ $uniq2 } > 1 ) #minus strand
             ) {
             print F2;
             $dups++;