PGR and breast cancer: There are qualifications with respect to this evidence: (i) due to extremely low levels of viral DNA, the identification of MMTV by PCR in human breast cancers is not consistent, (ii) the assessment of histological characteristics is subjective, and (iii) the assessment of staining used in immunohistochemistry is also subjective (but no less subjective than other analyses based on immunohistochemistry, such as ER and PR protein expression in breast cancer).