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In the multiple steps in cancer progression, microRNAs (miRNAs) play significant roles in each stage. Reports of considerable differences in expression levels of miRNAs between normal and malignant tissues are understandable considering miRNAs are key regulators of gene expression. Dysregulation of miRNA expression levels in neoplasia occurs because many miRNAs are located in “fragile sites”, which are frequently deleted in cancer. miRNAs are often down regulated in cancerous tissues and target oncogenic proteins are classified as tumour suppressor miRNAs, such as let-7. While, miRNAs that are frequently over-expression in neoplastic tissues compared to normal tissues and regulate tumour suppressor proteins are categorized as "oncomiRs". In this review, we summarize information about microRNAs involved in the emerging field of cancer stem cells, and microRNAs involved in breast cancer, an area of our expertise. The application of miRNAs to cancer therapeutics and diagnostics is emerging as an important field of gene therapy. The diverse nature of miRNAs in cancer is continually being elucidated to lead to the enigmatic treatment options for neoplastic disease.