Malignant melanoma, an aggressive and increasingly common cancer, is characterized by a strikingly high rate (70%) of mutations in BRAF, a key component of the mitogen-activated protein (MAP) kinase signaling pathway. How signaling events downstream from BRAF affect the underlying program of gene expression is poorly understood. We show that the Brn-2 POU domain transcription factor is highly expressed in melanoma cell lines but not in melanocytes or melanoblasts and that overexpression of Brn-2 in melanocytes results in increased proliferation. Expression of Brn-2 is strongly upregulated by Ras and MAP kinase signaling. Importantly, the Brn-2 promoter is stimulated by kinase-activating BRAF mutants and endogenous Brn-2 expression is inhibited by RNA interference-mediated downregulation of BRAF. Moreover, silent interfering RNA-mediated depletion of Brn-2 in melanoma cells expressing activated BRAF leads to decreased proliferation. The results suggest that the high levels of Brn-2 expression observed in melanomas link BRAF signaling to increased proliferation.The mitogen-activated protein (MAP) kinase signaling pathway, comprising the RAS-RAF-MEK-ERK cascade, plays a key role in the cellular response to extracellular signals, and deregulation of the pathway, for example, as a result of activating mutations in Ras, is a feature of many cancers. Three Raf genes have been described (ARAF, BRAF, and CRAF) that are regulated by interaction with Ras, and RAF protein can phosphorylate and activate the MAP kinase kinase MEK. The importance of this signaling pathway in cancer has recently been highlighted by the observation that almost 70% of melanomas and primary nevi exhibit kinase-activating mutations in BRAF (11,26). Understanding how signaling by the MAP kinase pathway affects the underlying program of gene expression is therefore a particularly important issue for melanoma, and it is presumed that deregulation of MAP kinase signaling results in altered expression or activity of key transcription factors, leading to aberrant growth control. The identification of BRAF target genes currently represents a major goal in melanoma biology.The POU domain transcription factor Brn-2 (also called N-Oct3 and POU3F2) (see reference 28 for a review) has been implicated in neuronal differentiation (16) and activation of the corticotropin-releasing hormone gene (22,29). Targeted disruption of the Brn-2 gene in the mouse results in loss of specific neuronal lineages in the hypothalamus and consequent loss of the posterior pituitary gland (25, 29). Brn-2-negative mice therefore die within 10 days after birth, although the specific cause of death is not apparent. Although Brn-2 clearly plays a major role in neuronal development, evidence also implicates it in melanoma growth and survival. Thus, Brn-2 mRNA is overexpressed in melanoma compared to normal melanocytes (15,33,34), and Brn-2 expression is substantially downregulated by differentiating agents (32). Importantly, downregulation of Brn-2 with an antisense strategy resulted in ...