A recent study aiming at finding new biomarkers, by a proteomic approach, for mitral regurgitation due to mitral valve prolapse in humans showed that haptoglobin, platelet basic protein, and C4b levels could be reduced in these subjects, and therefore, a possible role in the pathophysiology of mitral valve prolapse/mitral regurgitation has been hypothesized for these molecules (and pathways in which are involved) (104). The gene discussed is C4B; the disease is mitral valve insufficiency.