•Based on antibody-based proteomics by proximity extension assays, pre-treatment levels of several proteins were predictive of shorter progression-free survival (PFS) after treatment with MAPK-inhibitors in metastatic cutaneous melanoma (CM), including IL6, IL10, CCL-2/-3/-4, LGALS1, and CSF1. Here, LGALS1 is linked to cutaneous mastocytosis.