Furthermore, given that hPRLrI is expressed in a wide array of breast cancer cell lines (Supplementary Fig. 4a), patient-derived xenografts (PDX; Supplementary Fig. 4b), and is not detectable at the protein level in either normal breast MCF10A cells or the partially-transformed derivative MCF10AT (Supplementary Fig. 4c, Fig. 1a), the effect of hPRLrI loss on breast cancer cells was examined in order to validate the oncogenic significance of this isoform in a more progressive stage of the disease21. Here, PRLR is linked to breast cancer.