To explain the significance of this suppressive effect, we turn to Engel et al. 47 who reported that NK cell-induced tumor cell lysis was significantly more pronounced in triple negative breast cancer cells, where estrogen receptor (−) and progesterone receptor (−) did not over-express the HER-2 receptor, compared to ER- positive MCF7 cells47, indicating that MCF7 cells are prone to resist NK cell cancer surveillance. This evidence concerns the gene PGR and cancer.