processes, including inflammation, bone homeostasis, neurological development and
coagulation. TREM-1, the first to be identified, acts to amplify inflammation. Other TREM
proteins regulate the differentiation and function of macrophages, microglia, dendritic cells,
osteoclasts and platelets. Here we discuss the state of the field, putative ligands of TREM
proteins and the challenges that remain in understanding TREM biology.