Abstract： Objective To investigate the expression and biological significance of PIM1 in the breast cancer,atypical hyperplasia breast,and normal breast tissues.Methods The protein expression levels of PIM1 in the breast cancer,atypical hyperplasia breast,and normal breast tissues were assayed by immunohistochemistry and Western blot.Results The expression rates of PIM1 protein were 78.75％ (63/80)in breast cancers,42.00％ (21/50) in atypical hyperplasia tissues,and 23.33 ％ (7/30) in normal breast tissues,respectively.A significant correlation was found between PIM1 expression and the clinical stage and lymphonodus metastasis.However,no significant correlation was found between PIM1 expression and patients'age and tumor sizes.The level of PIM1 protein in breast cancers was obviously higher than that in non - cancerous tissues.Conclusions The positive rate of PIM1 in breast cancers is significantly higher than that in atypical hyperplasia breast tissues and in normal breast tissues.PIM1 may be an early molecular event in mammary tumorigenesis,and its overexpression may significantly relate to the malignant progression.It would be a new parameter for the early diagnosis and a biomarker for breast cancers.It is feasible to utilize paraffin specimens for index test with advantages of convenience,easy availability,and low expense.