Stromal CD26 expression is known to be barely detectable in different human SCCs.17, 18 However, whether stromal CD26 expression is also downregulated in human breast carcinomas remains controversial; stromal CD26 expression was reportedly undetectable in a tumor taken from one breast cancer patient,20 while its expression was detected in another human breast carcinoma.21 Moreover, particular stromal cell types potentially lacking CD26 expression in tumors and the molecular mechanisms underlying the decreased stromal CD26 expression have not as yet been fully elucidated. Here, DPP4 is linked to breast cancer.