@seq = split(//,$string);
   $i = 0;
   foreach$letter(@seq){
      switch($letter){
         case "A" {  $A[$i]++;}
         case "C" {  $C[$i]++; }
         case "G" {  $G[$i]++;}
         case "T" {  $T[$i]++; }
         else{$N[$i]++;}
      }
      $i++;
   }