Baseline patient samples were classified as responders (n = 9) or non-responders (n = 15) based on the response at 3 months post ICI-treatment….PAK4 was found to be elevated in tumor samples from non-responder patients by both unbiased proteomics analysis...Analysis of the 24 FFPE samples from metastatic melanoma patients treated with ICI therapy stratified into responders and non-responders lead to the identification of PAK4 as a potential marker for poor response to immunotherapy in melanoma patient samples.