Abstract Background: Pakistan has the second highest rate of breast cancer in Asia after Israel with nearly 1 out of 9 women at risk of developing breast cancer at some stage in their lifetime. A potential role of viruses in human breast cancer induction/development is increasingly coming to surface. Several different groups around the world have demonstrated the presence of mousemammary tumor virus (MMTV)-like sequences in tumor but not normal breast tissue. However, these findings are controversial and seem to differ depending upon the geographic location and ethnicity of a population. Some find MMTV-like sequences in tumors of breast cancer patients only, while others believe these results are due to PCR contamination. Yet others believe that these sequences are real and represent endogenous retroviruses that reside within the human genome. Methods: These possibilities were tested by collecting blood and breast tissue samples from breast cancer and normal individuals after informed consent and ethical approval. A total of 146 blood and tumor samples from cancer patients and 164 blood genomic DNA samples from healthy individuals were collected and subjected to PCR. Primers for different regions of the virus were designed in such a manner that they encompassed regions conserved among different MMTV strains, but were different at their 3’ ends from the human endogenous virus K (HERV-K) that has -50% homology to MMTV. Results: Single PCR screening of all samples gave sporadic, mostly weak positive results. However, nested PCR of a subset of the samples from normal and cancer patients revealed that for the pol region, over 50-100% of the samples were positive, for env 15-75% of the samples were positive, while for long terminal repeat (LTR) 5-100% of the samples were positive, depending upon whether they were from blood or breast tissue. Sequencing of the PCR fragments further revealed these sequences to be 90-100% homologous to Mtv-8 but not HERV-K, thus identifying these amplified bands to be of MMTV origin. Finally, test of the wild Pakistani Mus musculus revealed that they contain endogenous MMTVs very similar to Mtv-8. Conclusions: Together, these unexpected results suggest that the Pakistani population may be exposed to MMTV, maybe through zoonotic transmission from mice. These observations need further stringent study and confirmation. Differential expression studies are in the process as well as hunt for viral integration sites. It is only with a positive demonstration of integration sites in normal individuals that one can definitively prove whether MMTV is actually in the human population. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P6-06-04.