Paraffin-embedded consecutive slides of normal tissues (n=30) and breast cancer tissues (n=30) were obtained from Shanghai Outdo Biotech Co., Ltd (Shanghai, China). For in situ detection of miR-20a, the slides were hybridized with DIG-labeled oligonucleotide (Exiqon) complementary to miR-20a. Hybridization was visualized by nitroblue tetrazolium/5-bromo-4-chloro-3-indolyl phosphate color substrate (Roche, Basel, Switzerland). The staining intensity of miR-20a was determined using a visual grading system.