PRLR and breast cancer: An anti-PRLR ADC comprised of a high-affinity function blocking anti-PRLR IgG1 antibody (REGN2878) conjugated to the cytotoxic maytansine derived DM1, has shown significant anti-tumor activity against several breast cancer xenograft tumor models although the activity was limited to those models with high levels of PRLR expression [4].