* Corresponding author: sirha@ dtu. dk. AUTHOR CONTRIBUTIONS Conceptualization: UKH, RL, PTS, SRH; Formal Analysis: UKH, RL; Funding Acquisition: PN, SRH; Investigation: UKH, RL; Methodology: UKH, RL, DI, MV; Project Administration: SRH; Resources: DI, CC, MV, JCB, PTS, PN, SRH; Supervision: PN, SRH; Validation: UKH, RL; Visualization: UKH, RL; Writing-Original Draft Preparation: UKH, SRH; Writing-Review and Editing: UKH, CC, JCB, PTS, PN, SRH

Ethics statement Samples of all patients with Merkel cell carcinoma (MCC) were kindly provided by the Fred Hutchinson Cancer Research Center (Seattle, WA) under approval by their institutional review board. The sample collection complied with the Declaration of Helsinki. Blood samples were collected from 49 patients at the time of diagnosis and tumor samples were collected from three patients. Blood samples from 54 healthy donors were collected from the blood bank at Rigshospitalet (Copenhagen, Denmark) with approval from the local Scientific Ethics Committee of the Capital Region, Denmark. Written informed consent was obtained from all patients and donors according to the Declaration of Helsinki.