Breast cancer is the most frequent malignancy in women and has devastating effects on both their physical and emotional wellbeing. More cases of it are expected to surface in the future. Mounting data shows that microRNAs (miRNAs) play critical roles in carcinogenesis and development. This study highlights the contrasting roles of miR-9 in breast cancer, which is important since miR-9 plays a crucial role in both the development and the progression of the disease. The major focus of this article is a brief summary of miR-9's functions in the development of breast cancer.