code: function getRefSeq(currentSequence, min, max) {
      var refSeq = [];
      if (currentSequence) {
          var csStart = currentSequence.start|0;
          var csEnd = currentSequence.end|0;
          if (csStart <= max && csEnd >= min) {
              var sfMin = Math.max(min, csStart);
              var sfMax = Math.min(max, csEnd);
  
              for (var i = 0; i < sfMin - min; i++)
                  refSeq.push('N');
              refSeq.push(currentSequence.seq.substr(sfMin - csStart, sfMax - sfMin + 1));
              for (var i = 0; i < max - sfMax; i++)
                  refSeq.push('N');
          }
      }
      return refSeq.join('');
  }
docs: Constructs the reference sequence for a given window.
  
  Params
  currentSequence: DasSequence object containing ref sequence
  in current browser view.
  min, max: min and max position for window.
  
  Returns a string containing the refseq, padded with 'N' where sequence is not
  available.