Abstract Signaling by polypeptide hormone prolactin (PRL) is mediated by its cognate receptor (PRLr). PRLr is commonly stabilized in human breast cancer due to decreased phosphorylation of residue Ser349, which when phosphorylated recruits the βTrcp E3 ubiquitin ligase and facilitates PRLr degradation. Here, we show that an impaired PRLr turnover results in an augmented PRL signaling and PRL-induced transcription. Human mammary epithelial cells harboring degradation-resistant PRLr display accelerated proliferation and increased invasive growth. Conversely, a decrease in PRLr levels achieved by either pharmacologic or genetic means in human breast cancer cells dramatically reduced transformation and tumorigenic properties of these cells. Consequences of alteration of PRLr turnover for homeostasis of mammary cells and development of breast cancers, as well as the utility of therapies that target PRLr function in these malignancies, are discussed. [Cancer Res 2009;69(7):3165–72]