Background: mTOR inhibitors have clinical utility in VEGF-refractory RCC and may be useful in non-clear cell disease. In general, mTOR inhibition is inferior to targeting VEGF in the treatment-naïve setting. However, some patients clearly demonstrate mTOR-dependent disease. It is critical that new biomarkers are identified to optimize patient selection for mTOR-targeting therapeutics. One such potential marker, the mTOR autophosphorylation site, S2481, is a direct readout of mTOR activation. The objective of this pilot study was to analyze the levels of phospho-S2481 in different RCC subtypes. Methods: Utilizing standard immunohistochemistry methods, archived, formalin-fixed paraffin-embedded primary tumor samples from an existing tissue microarray of patients with clear cell (cc, n=203), chromophobe (n=21) and papillary RCC (n=38) were stained for the presence of phospho-S2481. Samples from transitional cell carcinoma cases (n=38) were included as a control. Mean scores from the four cancer subtypes were calculated, and the Kruskal-Wallis test was applied to demonstrate the statistical relevance of the mean scores. Covariates such as stage, grade, age, and tumor size were analyzed by logistic regression. Results: ccRCC patients were most likely to have high levels of phospho-S2481 staining (mean=2.08), while the TCC controls had the lowest amount (mean=1.11), with papillary and chromophobe falling in between (mean=1.41 and 1.38 respectively). The mean score difference among the four cancer subtypes was highly significant (Chi-square statistic: 54.51, p <.0001). Tumor size and cancer subtype were highly correlated with the score (p<.01). Comparison of the odds ratios for the three RCC subtypes demonstrated that ccRCC cases were three times more likely to have high levels of phospho-S2481 and four times more likely than papillary RCC (p<.05). Conclusions: Phospho-S2481 may have the ability to classify RCC patients of different subtypes. Exploration of phospho-S2481’s ability to predict patient response to mTOR inhibitors is under way.