Intriguingly, there was a correlation between Arg1 expression and Trem2, and thus, Katzenelenbogen et al. showed that Arg1+Trem2+ TAMs and Arg1+Trem2+ monocytic cells represented key regulatory myeloid subsets.1 To further understand the role of Trem2, Katzenelenbogen et al. challenged wild-type mice and Trem2−/− mice with MCA205 tumor cells. Here, ARG1 is linked to neoplasm.