目前推測許多環境因子皆可能為乳癌的形成原因，其中包括老鼠乳腺腫瘤病毒(mouse mammary tumor virus, MMTV)。老鼠乳腺腫瘤病毒為致癌B型反轉錄病毒，傳染途徑分為內源性及外源性感染，皆能誘導小鼠乳癌之形成。已有許多研究報告發現人類乳癌中高度表現類似老鼠乳腺腫瘤之病毒序列，由此顯示老鼠乳腺腫瘤病毒與人類乳癌的關係。遂有假說推斷因犬貓經常與人類接觸，而從老鼠媒介MMTV或MMTV-like virus傳播至人類。但目前尚無實驗室研究犬貓之乳癌組織中是否有老鼠乳腺腫瘤病毒或類似老鼠乳腺腫瘤病毒之序列。由於此假說仍未有進一步的研究證實，遂本實驗之研究目的為使用nested PCR方法，偵測犬貓乳腺腫瘤組織是否有類似老鼠乳腺腫瘤病毒之序列。若能從組織中偵測到類似序列，將會與老鼠乳腺腫瘤病毒比對序列相似性。另外同時檢視臨床與病理因子,進行多重分析,觀察結果是否與序列的出現有相關性。 實驗樣本的取得是來自中興大學獸醫教學醫院 (於西元1995年至2008年) 經由外科手術切除之乳腺腫瘤組織，來自113隻狗及11隻貓，結果發現在MMTV-like env and LTR gene sequences在犬惡性乳癌組織中的出現比例分別為3.49%及18.6%，而在貓惡性乳腺腫瘤組織中的出現比例分別為22.2%及 22.2%；DNA定序結果，與陽性對照組及MMTV的基因相似性有94–98 %，且在犬隻偵測的MMTV-like LTR gene sequences與MMTV的基因相似性較低 (大約89%)。另外，可以在犬貓正常乳腺組織中偵測出序列之動物，其鄰近之乳腺腫瘤也同樣偵測出序列，遂推測此序列或許為內源性來源。我們無法排除此序列為來自老鼠之可能性，目前也仍不清楚犬貓從老鼠媒介MMTV或MMTV-like virus之傳染途徑，因此仍須進一步證實犬貓乳腺腫瘤組織中是否出現老鼠乳腺腫瘤病毒顆粒。
Mouse mammary tumor virus (MMTV), an oncogenic B retrovirus, contributes to occurrence of breast cancer in mice, acquired horizontally through the milk (exogenous) to susceptible offspring or vertically through the germ line (endogenous provirus). The MMTV-like sequences have been found in human breast cancer, implicating the involvement of MMTV in human breast cancer. Dogs and cats with frequent human contacts were proposed to contribute to transmission of this virus between mouse and man; however, this hypothesis has not yet been defined. Hence, the goal of this study was to detect MMTV-like gene sequences in canine and feline mammary tumors. MMTV-like env and LTR sequence were amplified by means of nested PCR. The sequences of resultant DNA fragments were verified by direct automated sequencing. Moreover, we also examine clinical and pathological parameters by multivariate analysis, and find the association with the presence of the MMTV-like sequences. In total of 124 samples from 113 dogs and 11 cats who had undergone surgery were obtained from 1995 to 2008 in Veterinary Medical Teaching Hospital, National Chung Hsing University. Specific MMTV-like env and LTR sequences were found in 3 (3.49%) and 16 (18.6%) of dogs with malignant mammary tumors, respectively. Besides this, specific MMTV-like env and LTR sequences were also detected in 2 (22.2%) and 2 (22.2%) of feline malignant mammary tumors, respectively. The homology of these sequences to MMTV identified from NIH 3T3 (MMTV-positive murine cell line) and MMTV-like sequences approximately range from 94% to 98%; however, a lower similarity (approximately 89%) was observed when MMTV-like LTR sequences from canine were aligned with those of MMTV identified from mouse (HeJ strain). Moreover, sequences of LTR present in 2 dogs (13.3%, 2/15) and sequences of LTR present in one cat (50.0%, 1/2), both in mammary tumor and in mammary normal tissue suggesting that it may be an endogenous origin. Nevertheless, it can’t be ruled out completely the possibility of the transmission from mice. The role of dog and cat plays in transmission of MMTV from mice to human remains unclear. In this respect, isolation of MMTV particles from such animals would be crucial.