MicroRNA-21 targets the tumor suppressor gene tropomyosin 1 (TPM1). This study demonstrates that mir-21 down-regulates TPM1 expression, which may explain why suppression of mir-21 inhibits tumor growth. The study used two-dimensional difference in-gel electrophoresis (2-DIGE) to identify TPM1 as a potential mir-21 target. The 3'-untranslated region (3'-UTR) of TPM1 variants V1 and V5 contains a putative mir-21 binding site. Luciferase assays confirmed that mir-21 reduces luciferase activity, while anti-mir-21 increases it, indicating that the mir-21 binding site is critical for regulation. Western blot analysis showed that TPM1 protein levels are regulated by mir-21, while no difference was observed at the mRNA level, suggesting translational regulation. Overexpression of TPM1 in breast cancer MCF-7 cells suppressed anchorage-independent growth, supporting the role of TPM1 as a tumor suppressor. These findings suggest that mir-21 functions as an oncogene by targeting TPM1. The study also highlights the importance of understanding miRNA target genes, as miRNAs regulate a large fraction of protein-coding genes. The results provide insights into the molecular mechanisms of miRNA-mediated gene regulation and its impact on tumor growth.MicroRNA-21 targets the tumor suppressor gene tropomyosin 1 (TPM1). This study demonstrates that mir-21 down-regulates TPM1 expression, which may explain why suppression of mir-21 inhibits tumor growth. The study used two-dimensional difference in-gel electrophoresis (2-DIGE) to identify TPM1 as a potential mir-21 target. The 3'-untranslated region (3'-UTR) of TPM1 variants V1 and V5 contains a putative mir-21 binding site. Luciferase assays confirmed that mir-21 reduces luciferase activity, while anti-mir-21 increases it, indicating that the mir-21 binding site is critical for regulation. Western blot analysis showed that TPM1 protein levels are regulated by mir-21, while no difference was observed at the mRNA level, suggesting translational regulation. Overexpression of TPM1 in breast cancer MCF-7 cells suppressed anchorage-independent growth, supporting the role of TPM1 as a tumor suppressor. These findings suggest that mir-21 functions as an oncogene by targeting TPM1. The study also highlights the importance of understanding miRNA target genes, as miRNAs regulate a large fraction of protein-coding genes. The results provide insights into the molecular mechanisms of miRNA-mediated gene regulation and its impact on tumor growth.