I don’t have a conclusive answer to your question, but I can speculate. Upon entry into a cell, the RNA genome of retroviruses is retrotranscribed with reverse transcriptase into DNA. It must then complete the difficult task of crossing the nuclear membrane in order to integrate into host cell DNA within the nucleus. Retroviral nuclear entry has been studied extensively only in murine leukemia virus (MLV), an oncogenic gammaretrovirus, and HIV-1, a lentivirus (reviewed in Metreyek et al., Viruses 5:2483-2511, 2013; Kobiler et al., Nucleus 3:526-539, 2012; Suzuki et al., Nat Rev Microbiol 5:187-196, 2007). Studies on MLV, indicate that cell mitosis (with temporary dissolution of the nuclear membrane) is necessary for retroviral entry into the nucleus. If mitosis does not occur, the unintegrated forms remain in the cytoplasm. The more complex HIV-1 is able to enter the nucleus in the absence of mitosis, by a mechanism not completely elucidated. The mechanism of nuclear entry in the oncogenic deltaretrovirus group, to which BLV belongs, has not been studied, but presumably would be more like MLV. I would speculate that the cells pictured in our publication, were perhaps not dividing. The donor of the tissue was 50 years old, so likely low in the steroid hormones that would stimulate mammary epithelial cell division during pregnancy, and lactation. The specimen was obtained during reduction mammaplasty surgery; the donor had no breast malignancy or history of breast cancer. Even though BLV in this situation may not be active, it serves as a biomarker for exposure to the virus, which is useful for epidemiologic studies.