code: def _get_nblock_regions(in_file, min_n_size, ref_regions):
      
      out_lines = []
      called_contigs = set([])
      with utils.open_gzipsafe(in_file) as in_handle:
          for line in in_handle:
              contig, start, end, ctype = line.rstrip().split()
              called_contigs.add(contig)
              if (ctype in ["REF_N", "NO_COVERAGE", "EXCESSIVE_COVERAGE", "LOW_COVERAGE"] and
                    int(end) - int(start) > min_n_size):
                  out_lines.append("%s\t%s\t%s\n" % (contig, start, end))
      for refr in ref_regions:
          if refr.chrom not in called_contigs:
              out_lines.append("%s\t%s\t%s\n" % (refr.chrom, 0, refr.stop))
      return pybedtools.BedTool("\n".join(out_lines), from_string=True)
docs: Retrieve coordinates of regions in reference genome with no mapping.
      These are potential breakpoints for parallelizing analysis.