439 Background: Renal cell carcinoma (RCC) represents 2 to 3% of all cancers of the Western countries. Currently, sunitinib, an anti-receptor tyrosine kinase (RTK), particularly of PDGFR and VEGFR, is the first-line therapy for metastatic RCC (mRCC), with significant improvement in clinical outcome. However, there is a lack of predictive biomarkers of sunitinib response. Recently, we and others have suggested that the RTK AXL may modify the response to sunitinib. Objective: To study the expression of AXL in a series of mRCC patients treated with sunitinib and to correlate it with patient’s clinico-pathological features and therapeutic response. Methods: Sixty four mRCC patients (51 clear cell carcinomas and 13 non-clear cell carcinomas) were evaluated and AXL expression was assessed by immunohistochemistry in TMAs. Results: AXL positivity was observed in 47% of cases. The multivariate survival analysis showed that AXL expression was a poor prognostic factor in patients with mRCC (HR 2.007, 1.006 - 4.006, p=0.048). In addition to the positivity for AXL, other prognostic factors were absence of nephrectomy, more than one site of metastasis, and bone metastasis. Considering only the clear cell subtype, the univariate analysis showed that AXL expression was statistically associated with a poor prognosis, with a median overall survival of 13 months vs. 43 months in patients with negative AXL. In this subtype, along with the AXL positivity, other prognostic factors were absence of nephrectomy, KPS, more than one site of metastasis, and liver metastasis. Moreover, AXL expression was associated with shorter progression to sunitinib. Conclusions: AXL expression was correlated with worse clinical outcome and may be an important prognostic biomarker in sunitinib-treated metastatic renal cell carcinoma patients.