This study investigates the role of microRNA-21 (miR-21) in cell invasion and tumor metastasis. MiR-21 is known to function as an oncogene and regulate tumor suppressor genes such as tropomyosin 1 (TPM1). The authors found that suppressing miR-21 in metastatic breast cancer MDA-MB-231 cells significantly reduced invasion and lung metastasis. They identified two additional direct targets of miR-21, programmed cell death 4 (PDCD4) and maspin, both of which are involved in invasion and metastasis. Overexpression of TPM1, PDCD4, and maspin reduced cell invasion, and the expression of these genes inversely correlated with miR-21 levels in human breast tumor specimens. These findings suggest that miR-21 plays a crucial role in tumor growth, invasion, and metastasis by targeting multiple tumor suppressor genes. Therefore, targeting miR-21 may provide a novel approach for treating advanced cancers.This study investigates the role of microRNA-21 (miR-21) in cell invasion and tumor metastasis. MiR-21 is known to function as an oncogene and regulate tumor suppressor genes such as tropomyosin 1 (TPM1). The authors found that suppressing miR-21 in metastatic breast cancer MDA-MB-231 cells significantly reduced invasion and lung metastasis. They identified two additional direct targets of miR-21, programmed cell death 4 (PDCD4) and maspin, both of which are involved in invasion and metastasis. Overexpression of TPM1, PDCD4, and maspin reduced cell invasion, and the expression of these genes inversely correlated with miR-21 levels in human breast tumor specimens. These findings suggest that miR-21 plays a crucial role in tumor growth, invasion, and metastasis by targeting multiple tumor suppressor genes. Therefore, targeting miR-21 may provide a novel approach for treating advanced cancers.