en Original/Research Article <div style="text-align: justify;"><strong>Abstract<br> Introduction:</strong> Bovine leukemia virus (BLV) causes enzootic bovine leukemia, andis closely related to human T-lymphotropic virus type 1. It expresses microRNAs of unknown function and codes Tax, the protein that mediates malignant transformation. BLV is capable of infecting B- and T-lymphocytes, endothelial cells, and mammary epithelial cells of cattle. Several studies demonstrated the presence of BLV DNA in human tissue, and it is significantly associated with breast cancer in case-control studies using in situ PCR, a highly sensitive and specific technique. The current study was the first report of BLV DNA detection by in situ PCR in tissue from Argentinian females with a diagnosis of breast cancer.<br> <strong>Methods:</strong> In situ PCR was done to detect BLV DNA in 85 human FFPE breast cancer tissue samples. The association of BLV DNA and expression of Ki67 and Her-2 was assessed.<br> <strong>Results:</strong> BLV DNA presence could be determined in 22.6% of the analyzed samples, and its presence was associated with an increase of the expression and prognosis markers Ki67 (P=0.009) and HER-2 (P=0.044) determined by conventional immunohistochemistry. No statistical significance was observed between the presence or absence of hormonal receptors and the presence of BLV DNA.<br> <strong>Conclusions:</strong> The obtained results support the idea that BLV might play a role in malignant tissue transformation.</div> Leukemia Virus, Bovine, Human Breast Cancer, Direct In Situ PCR, Cell Proliferation, Prognosis 16 24 http://mcijournal.com/browse.php?a_code=A-10-240-1&slc_lang=en&sid=1 Maria Carolina Ceriani cceriani@vet.unicen.edu.ar 10031947532846001877 10031947532846001877 Yes FCV-UNCPBA-CONICET Pamela Anahi Lendez palendez@vet.unicen.edu.ar 10031947532846001878 10031947532846001878 No FCV-UNCPBA Lucia Martinez Cuesta luciamartinezcuesta@hotmail.com 10031947532846001879 10031947532846001879 No Laboratorio de Virología, Facultad de Ciencias Veterinarias, UNCPBA-CONICET Maria Victoria Nieto Farias vnieto@vet.unicen.edu.ar 10031947532846001880 10031947532846001880 No Laboratorio de Virología, Facultad de Ciencias Veterinarias, UNCPBA-CONICET Hua Min Shen huamin@berkeley.edu 10031947532846001881 10031947532846001881 No The School of Public Health, University of California, Berkeley, California, USA Gertrude Case Buehring buehring@berkeley.edu 10031947532846001882 10031947532846001882 No The School of Public Health, University of California, Berkeley, California, USA Guillermina Laura Dolcini gdolcini@vet.unicen.edu.ar 10031947532846001883 10031947532846001883 No Laboratorio de Virología, Facultad de Ciencias Veterinarias, UNCPBA-CONICET