The role of the polypeptide hormone prolactin (PRL) in the pathogenesis and progression of human breast cancer has become established in recent years. Acting at the autocrine/paracrine level, PRL functions to stimulate the growth and motility of human breast cancer cells. These actions require the presence of the transmembrane prolactin receptor (PRLr). The classic belief is that the PRLr functions "at a distance" through the activation of signaling networks. However, numerous reports now indicate that intact transmembrane receptors localize to the nucleus and directly contribute to transcriptional activation. Yet, without DNA binding capabilities or intrinsic chromatin remodeling activity, the mechanism by which these receptors serve to regulate transcription remains elusive. In this thesis, we investigate the function of nuclear PRLr, given preliminary observations of its nuclear presence. For the first time, we find that the prolactin receptor (PRLr) localizes to the nucleus where it functions as a co-activator through interaction with the latent transcription factor Stat5a and the high mobility group N2 protein (HMGN2). We identify a novel transactivation domain (TAD) within the PRLr and demonstrate that it is activated by ligand-induced phosphorylation, an event coupled to HMGN2 binding. Depletion of HMGN2 reduces binding of PRLr and Stat5a to chromatin, and results in impaired Stat5a-drive gene expression. We propose that the PRLr-mediated recruitment of HMGN2 facilitates PRLr/Stat5a assembly onto chromatin, thus enabling Stat5a-driven transcription. These data are of significant importance to the broad field of signal transduction as they expand the growing field of cell surface receptor nuclear localization to include a novel receptor. In addition, these studies are of note as they elucidate a molecular mechanism though which cell surface receptor-mediated transcriptional regulation is achieved. A direct pathophysiologic relevance is demonstrated through observations that the PRLr TAD is necessary for in vitro anchorage-independent growth and is increasingly activated as a function of neoplastic progression in vivo. Additionally, these studies reveal that the PRLr TAD contributes to the expression of the estrogen receptor and progesterone receptor, whose quantification has proven to be extremely useful in the prognosis of breast cancer. Our data, encompassing the mechanistic, functional and biological actions of nuclear PRLr, provide key insights into full-length cell surface receptor nuclear translocation, a phenomenon that has remained elusive for nearly two decades.