A third of breast cancers are estrogen dependent and respond to endocrine therapy. The estrogen receptor (ER) was the first marker used to predict the responses to treatment, and two-thirds of ER positive tumors show a favourable response. Several estrogen-regulated proteins were further studied in a search to enhance the prediction accuracy of ER status: progesterone receptors, 24-K heat shock protein , cathepsin D . and recently pS2 protein. The pS2 gene, also named BCEI , pNR-2 , Md2 , was first identified by two groups using differential screening of a complementary DNA library derived from a human breast carcinoma cell line (MCF-7) grown with and without estrogens [3,6]. Later on two independent English groups [4,5] and a Japanese group  identified a gene similar to pS2. The pS2 mRNA, relatively abudant (0.8%) in the MCF-7 cell line when stimulated by estrogens, encodes a cystein-rich, 84 aminoacids peptide which is secreted by breast cancer cells. The expression of the pS2 gene, pS2 protein assays in tumor cytosols and more recently pS2 detection by immunocytochemistry, have been described in several series of breast cancers [5,8-18]. © 1993.
Pichon, M. F., & Milgrom, E. (1993). Clinical significance of the estrogen regulated pS2 protein in mammary tumors. Critical Reviews in Oncology and Hematology. https://doi.org/10.1016/1040-8428(93)90017-X