TGFB1 and neoplasm: Given the induction of the attenuated CD26 expression on TGF‐β‐treated human mammary fibroblasts (Figure 3G‐I), different sources of TGF‐β derived from tumor cells and stromal cells37 in addition to CAFs may contribute to inducing downregulation of CD26 expression on mammary fibroblasts during tumor progression.