Seeking an explanation for the observed discordance between Arg-1 levels in tumor and plasma, we hypothesized that tumor-derived exosomes present in patients’ plasma could be a major but silent (i.e., exosomal Arg-1 cannot be detected when measuring plasma Arg-1 as it is shielded by the exosomal membrane) contributor to high plasma Arg-1 levels in cancer patients. Here, ARG1 is linked to cancer.