Immune checkpoint inhibitors (ICI) have revolutionized therapy for advanced cancer, however many patients still do not respond to treatment. However, the efficacy and effectiveness of these therapies varies greatly across individual patients and among different tumour types. A substantial unmet need is thus the development of biomarkers of response to ICI, in order to identify, before initiation of treatment, which patients are likely to experience a response to and clinical benefit from such treatments. Here, the inventors analyzed SPHK1 mRNA in tumor biopsies by in situ hybridization using the RNAscope technology in a cohort of 32 patients suffering from metastatic melanoma. They showed that elevated expression of SPHK1, encoding sphingosine kinase 1 (SK1), which produces the oncometabolite sphingosine-1-phosphate (S1P) is associated with a poor survival in metastatic melanoma patients treated with to the well-known immune-checkpoint inhibitor anti-PD-1 antibody. Accordingly, the present invention relates to the use of SK1 as biomarker for predicting response to immune-checkpoint inhibitors.