However, it is difficult to find such a marker as available as HLA-B*27, because the markers of multifactorial diseases have either a much stronger association that assimilate them to monogenic diseases (e.g., BRCA1); or a much lower association and then they are used only in very rare cases or within research protocols (TCF7L2 in diabetes, for example). This evidence concerns the gene TCF7L2 and diabetes mellitus.