def _vars_type(rna_calls):
    """Given a list of VCF record objects from a VarScan output,
    returns a string denoting the zygosity composition of the calls."""
    het_count, hom_count = 0, 0
    for rna_call in rna_calls:
        if rna_call.samples[0].is_het:
            het_count += 1
        else:
            hom_count += 1

    if het_count == 0:
        assert hom_count != 0
        return 'ALL_HOM'
    elif hom_count == 0:
        assert het_count != 0
        return 'ALL_HET'
    return 'MIX'