The adult heart is resistant to cancer formation and the metastatic invasion of distant neoplasms. This biological advantage may be dictated by the molecular properties of myocytes that constitutes 90% of the myocardium. We raised the possibility that microRNAs (miRs) highly expressed in myocytes (myomirs) may translocate via gap junctions to neighboring cancer cells, preventing their growth or inhibiting their survival. First, we established whether overexpression of myomirs interferes with the proliferation and death of MCF7 human breast cancer cells. Infection of MCF7 with lentiviruses carrying miR-1, miR-133a and miR-499 (miR-MCF7) resulted in a 5-fold decrease in Ki67 labeling and a 20% increase in the fraction of cells arrested at G0/G1. In contrast, TdT-positive apoptotic cells averaged 0.5% and did not differ in miR-MCF7 and control cells. To mimic the in vivo condition, EGFP-labeled MCF7 were co-cultured with myocytes and, 4 days later, the expression of myomirs was measured in FACS-sorted MCF7. With respect to baseline, co-cultured MCF7 showed 100-fold, 16-fold, and 27-fold increase in the expression of miR-1, miR-133a and miR-499, respectively. Co-culture of myocytes and MCF7 led to the formation of gap junctions made of connexin 43 (Cx43) and connexin 45 (Cx45). Silencing of Cx43 and Cx45 decreased significantly the expression of myomirs in co-cultured MCF7. Importantly, proximity of MCF7 to myocytes reduced markedly the growth rate of the cancer cells. Subsequently, 1 x 106 MCF7 or miR-MCF7 were injected subcutaneously in NOD-scid mice. At 5 weeks, the tumors developed from miR-MCF7 were 70% smaller than those originated from control MCF7. Two doses of breast cancer cells were injected intramyocardially to establish their in situ tumorigenic effects. Tumor formation was found in all hearts that received 1 x 106 MCF7. Conversely, mice injected with 1 x 105 cells did not show macroscopic evidence of neoplastic lesions. The lack of tumor development in the latter case is consistent with the ability of the heart to prevent neoplasm development when cancer cell colonization is not massive. Our findings document that miR-1, miR-133a and miR-499 translocate from myocytes to cancer cells via gap junctions, inhibiting tumor growth in vitro and in vivo.