SMAD4 and cancer: Subsequently, we investigated the PPI and PDI signals using cell lysates expressing SMAD4 R361H and R361C, two naturally occurring cancer-associated hotspot mutations in the MH2 domain that are known to impair the SMAD4–SMAD3 PPI (Alazzouzi et al., 2005; Blackford et al., 2009; Mo et al., 2022).