Background: Tremelimumab (Ticilimumumab, Pfizer), a monoclonal antibody targeting CTLA-4, a T cell inhibitory molecule, has shown activity in metastatic melanoma. Ipilimumab (Yervoy, BMS), another antibody targettingCTLA-4, improves survival relative to a peptide vaccine and is now FDA approved. A minority of patients will achieve durable tumor control with CTLA-4 blockade and biomarkers are urgently needed to identify those patients. Methods: 170 inflammatory, melanoma-specific and CTLA4-pathway related mRNA transcripts were measured using RT-PCR in pre-treatment peripheral blood samples from 218 patients with refractory melanoma receiving tremelimumab in a multi-center phase II study. A 2-class latent model yielded a risk score based on 4-genes that was highly predictive of survival (p<0.001), and was used to categorize patients into low, medium and high-risk groups. An independent cohort of 260 treatment naïve melanoma patients receiving tremelimumab as part of a multi-center phase III study was then used to validate the risk score as well as the 3 risk groups defined using the pre-specified cut-points. Results: There was no significant difference between the two cohorts in terms of age, gender, stage of disease or ECOG status. Median time of follow up was 297 days for the training cohort and 386 days for the validation cohort. 67% of patients in the training cohort and 70% of patients in the validation died during time of follow-up. Collectively, the ability of the 170 genes to predict survival exhibited a high degree of consistency across the cohorts (p < 0.001). A 4-gene model including cathepsin D (CTSD), Phopholipase A2 group VII (PLA2G7), Thioredoxin reductase 1 (TXNRD-1) and Interleukin 1 receptor associated kinase 3 (IRAK3) predicted survival in the validation cohort (p=0.001 by log rank test). Multivariable cox analysis showed that the 4-gene model added to the predictive value of clinical predictors (p<0.0001). Conclusions: Expression levels of CTSD, PLA2G7, TXNRD1, and IRAK3 in peripheral blood are predictive of survival in melanoma patients treated with ticilimumab (αCTLA-4). Blood mRNA signatures should be further explored to define patient subsets likely to benefit from immunotherapy.