Background: Breast cancer is the most common tumor among women. miR-638 has been demonstrated to play an important role in various cancers. 
Purpose: In this study, we aimed to investigate the function and prognostic value of miR-638 in breast cancer. 
Methods: Quantitative real-time polymerase chain reaction analysis was used to evaluate the expression of miR-638 in breast cancer tissues and cell lines. The correlation of miR-638 with clinicopathological features was analyzed using the chi-squared test. Kaplan–Meier survival analysis and Cox regression assay were performed to investigate the prognostic value of miR-638 in breast cancer patients. The effects of miR-638 on the biological behavior of breast cancer cells were evaluated using functional assays. 
Results: The expression of miR-638 was downregulated in breast cancer tissues and cell lines (all P <0.05). Decreased expression of miR-638 was significantly correlated with lymph node metastasis (P =0.015) and TNM stage (P =0.021). Patients with low miR-638 expression had shorter overall survival compared with those with high levels (Log-rank P =0.025). The miR-638 could be considered as an independent prognostic factor for the patients (HR =0.321, 95% CI =0.117–0.882, P =0.027). Downregulation of miR-638 was capable of promoting cell proliferation, migration, and invasion in MDA-MB-231 and MCF-7 cells. 
Conclusion: All the results indicate that miR-638 is a tumor suppressor in breast cancer and is involved in the progression of breast cancer. Thus, it may serve as a prognostic biomarker for breast cancer.
Keywords: miR-638, cell proliferation, migration, invasion, prognosis, breast cancer