PGR and breast neoplasm: In contrast to this, a research performed by Shekhar et al. showed that sensitive premalignant EIII8 and tumorigenic MCF-7 cells when co-cultured with the fibroblast derived from ER−/progesterone receptor (PR)− human breast tumors conferred the tamoxifen resistance independently of EGFR (Shekhar et al. 2007).