TP53BP1 and persistent truncus arteriosus: Subsequently, protein levels of several well-established senescence markers were measured by immunoblotting, including elevated p16Arc and p21Cip1 (Fig. 4d); 53BP1 and phospho-p53/total p53 ratio (Fig. 4e) were detected in WT TAC hearts but not in Igfbp7−/− TAC hearts.