Abstract We have previously demonstrated that increases in Wnt5A result in an increase in metastatic capacity of melanoma cells and a decrease in the melanoma differentiation antigens MART1 and gp100. These data have been supported by several other laboratories, giving rise to the phenotype switching model of melanoma progression. This model suggest that less aggressive melanomas express a proliferative gene expression signature governed by the transcription factor MITF, while more aggressive melanomas express an invasive gene expression signature, governed by Wnt5A (Hoek et al, 2010). We hypothesize that this phenotype switch can be influenced by the changing microenvironment and that ROR receptor expression, also influenced by the microenvironment, is a key metastatic biomarker in melanoma. The orphan tyrosine kinase receptors, ROR1 and ROR2, make up a superfamily of receptors that, until recently, had no known signal pathway association or ligand identification. ROR2 has since been shown to play a role in Wnt signal transduction and studies from our laboratory have demonstrated an important role for ROR2 in increasing the metastatic potential of melanoma cells. Very little remains know about ROR1 in melanoma. Here, we demonstrate that ROR1 expression is inversely correlated to that of ROR2 and decreasing ROR1 expression increases overall metastatic potential. In keeping with these data, reduction of ROR1 by siRNA demonstrates a decrease in MART1 and GP100, and motility, and an increase in the invasive capability of melanoma cells. In addition, siRNA knockdown of ROR1 expression increased ROR2, Wnt5A, and clathrin expression, and treatment with recombinant WNT5A decreased ROR1 expression, implying that downregulation of ROR1 may be an important event in increasing malignancy in melanoma. Further, ROR2 knockdown in cells that have high levels of Wnt5A, but low levels of ROR1, increases ROR1 expression. Based on this data, ROR1/ROR2 receptor expression appears to be involved in melanoma cell phenotype switching. As ROR2 has previously been shown to be induced by HIF1-α, we asked if this ROR phenotype switch was controlled, in part, by hypoxia. Our data suggests that cells can increase their ROR2 levels by exposure to hypoxia. This is concomitant with a decrease in proliferation and an increase in invasion and Wnt5A expression. Overall these data suggest that changes in the melanoma microenvironment, such as hypoxia, may result in a switch from ROR1 positivity to ROR2 positivity, which ultimately contributes to increases in metastatic potential. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr B2.