def create_MD_tag(reference_seq, query_seq):
    """Create MD tag
    Args:
        reference_seq (str) : reference sequence of alignment
        query_seq (str) : query bases of alignment
    Returns:
        md_tag(str) : md description of the alignment
    """
    no_change = 0
    md = []
    for ref_base, query_base in zip(reference_seq, query_seq):
        if ref_base.upper() == query_base:
            no_change += 1
        else:
            if no_change > 0:
                md.append(str(no_change))
            md.append(ref_base)
            no_change = 0
    if no_change > 0:
        md.append(str(no_change))
    return ''.join(md)