Using a conditional knockout approach, we previously demonstrated that the Janus kinase 2 (Jak2) is crucial for prolactin signaling and normal mammary gland development. Prolactin is suggested to synchronously activate multiple signaling cascades that emerge on the prolactin receptor. This study demonstrates that Jak2 is essential for the activation of Stat5 and expression of Cish, a Stat5-responsive negative regulator of Jak/Stat signaling. However, Jak2 is dispensable for the PRL-induced activation of c-Src, Fak, and the MAPK pathway. Despite activation of these kinases that are commonly associated with proliferative responses, the ablation of Jak2 reduces the multiplication of immortalized mammary epithelial cells. Our studies show that signaling through Jak2 controls not only the transcriptional activation of the Cyclin D1 gene but, more importantly, it regulates the accumulation of the Cyclin D1 protein in the nucleus by altering the activity of signal transducers that mediate the phosphorylation and subsequent nuclear export of Cyclin D1. In particular, the levels of activated Akt and inactive GSK-3beta (i.e. a kinase that regulates the nuclear export and degradation of Cyclin D1) are reduced in MECs lacking Jak2. The proliferation of Jak2-deficient MECs can be rescued by expressing of a mutant form of Cyclin D1 that cannot be phosphorylated by GSK-3beta and therefore constitutively resides in the nucleus. Besides discriminating Jak2-dependent and Jak2-independent signaling events emerging from the prolactin receptor, our observations provide a possible mechanism for phenotypic similarities between Cyclin D1 knockouts and females lacking individual members of the PRLR signaling cascade, in particular the PRLR, Jak2, and Stat5.