Placental immune editing switches (PIES) have not evolved to prevent or to cause cancer but to make feto-maternal immune tolerance possible, which is at the very core of our placental mammalian (‘Eutherian’) nature. Aggressive epithelial cancers might be an unfortunate ‘side effect’ of this highly sophisticated biological nature. Microenvironmental properties in the placenta and decidua are thought to be a key to feto-maternal immune tolerance. Recently, in 2016–2018, we published the first human genomic and epigenomic evidence of similar gene expression profiles in immune regulatory genes in cancer (primary lobular infiltrating breast cancer and ipsilateral axillary metastatic lymph nodes) and both placenta and decidua of the same young patient with breast carcinoma during pregnancy. These findings led us to speculate that ectopic expression, or repression, of ‘PIES’ might be used by cancer cells during carcinogenesis or cancer progression to elude immune vigilance in spite of tumour-associated antigens or evolving neo antigenic landscapes. Cancers are well known to frequently express embryonic antigens, such as carcinoembryonic antigen, used as cancer markers and detectable in the blood circulation, or to express ectopic hormones. Why should cancer cells invent de novo complex new immune suppression mechanisms, if they could simply use innate ones developed during the long-term evolution of placental mammals in order to hide fetal paternal antigens from the mother’s own immune system?

Monotremata (Prototheria-like Echidnas or Platypus Ornithoryncus) are nonplacental egg-laying mammals and, in spite of rudimentary breast epithelial ducts and lobules, they are seldom reported to suffer from aggressive breast cancers.