def write_umi_in_header(reads, umi):
    """
    Copy the UMI in the header separated by an _ to use later with UMI_tools.

    params: seq
    params: config
    params: adaptator
    """
    header = reads.header.split(' ')
    header[0] += '_' + str(umi['seq'])
    reads.header = ' '.join(header)
    return reads
