In cancer, miR-221 exerts context-dependent oncogenic/tumor-suppressive effects by targeting phosphatase and tensin homolog (PTEN), cyclin-dependent kinase inhibitor 1c (CDKN1C/p57), and BCL2 modifying factor (Bmf), thereby regulating cell proliferation, invasion, stemness, and resistance to cancer therapy; it also serves as a non-invasive biomarker for glioma, papillary thyroid carcinoma, and colorectal cancer. This evidence concerns the gene PTEN and cancer.