JAK2 and breast carcinoma: Prolactin receptor is found in up to 80% of breast cancer cells (7), where it couples to several signaling pathways, including Janus Kinase (JAK)-2/Signal Transducer and Activator of Transcription (STAT)-5, mitogen-activated protein kinase (MAPK), and phosphatidylinositol-3-OH-kinase (PI3K).