ANPEP and neoplasm: Because MEK/ERK signaling regulates DR4 expression in different tumor cell lines18, to study the correlation between CD13-inhibition and death receptors, we first detected the levels of CD13, DR4, DR5, ERK1/2, and phosphorylated ERK1/2 in 13 indicated tumor cell lines by Western blot analysis (Figure 1A), and calculated the relative protein levels (Supplementary Figure S1).