PRLR and breast cancer: MDA-MB-231 cells also express other markers of apocrine breast cancer [32] such as prolactin-induced protein, PIP, epidermal growth factor receptor, EGFR, and, at particularly high levels, 3-hydroxy-3-methylglutaryl-Coenzyme A reductase, HMGCR, but not growth hormone receptor, GHR, and prolactin receptor, PRLR.