Secretory leukocyte protease inhibitor (SLPI), a nonglycosylated, single-chain protein with a molecular weight of 11.7 kDa, was originally identified as a serine proteinase inhibitor with activities against a variety of proteases [7], and played an important role in the development and metastasis of multiple cancers, such as liver cancer [8], breast cancer [9], ovarian cancer [10] and gastric cancer [11]. Here, SLPI is linked to breast cancer.