code: def _slice_vcf_chr21(vcf_file, out_dir):
      
      tmp_file = os.path.join(out_dir, "chr21_qsignature.vcf")
      if not utils.file_exists(tmp_file):
          cmd = ("grep chr21 {vcf_file} > {tmp_file}").format(**locals())
          out = subprocess.check_output(cmd, shell=True)
      return tmp_file
docs: Slice chr21 of qsignature SNPs to reduce computation time