We have demonstrated that (i) MMTV positive mouse mammary tumours are all synaptophysin and chromogranin positive (7 of 7 specimens – 100% positive), (ii) in MMTV positive human breast cancers only 3 of 22 (14%) are synaptophysin or chromogranin positive, (iii) there was no expression of synaptophysin and chromogranin in the comparative normal breast specimens and (iv) MMTV positive (and either synaptophysin and chromogranin positive or negative) human breast cancers have similar histological characteristics to neuroendocrine human breast cancers and to MMTV positive mouse mammary tumours. Here, SYP is linked to breast cancer.