docs: def get_gtf():
      """
      Download GTF file from Ensembl, only keeping the chr17 entries.
      """
      size, md5, url = GTF
      full_gtf = os.path.join(args.data_dir, os.path.basename(url))
      subset_gtf = os.path.join(
          args.data_dir,
          os.path.basename(url).replace('.gtf.gz', '_%s.gtf' % CHROM))
  
      if not _up_to_date(md5, subset_gtf):
          download(url, full_gtf)
          cmds = [
              'zcat', '<',
              full_gtf,
              '|', 'awk -F "\\t" \'{if ($1 == "%s") print $0}\''
              % CHROM.replace('chr', ''),
              '|', 'awk \'{print "chr"$0}\'', '>', subset_gtf]
  
          logged_command(cmds)