Immune responses are regulated by opposing positive and negative signals triggered by the interaction of activating and inhibitory cell surface receptors with their ligands. Shibuya et al. identified novel paired activated and inhibitory immunoglobulin-like receptors, designated myeloid-associated immunoglobulin-like receptor (MAIR) I and MAIR-II, whose extracellular domains are highly conserved by each other. MAIR-I, expressed on the majority of myeloid cells, including macrophages, granulocytes, mast cells, and dendritic cells, contains the tyrosine-based sorting motif and the immunoreceptor tyrosine-based inhibitory motif-like sequences in the cytoplasmic domains. On the other hand, MAIR-II, expressed on subsets of peritoneal macrophages and B cells, associates with the immunoreceptor tyrosine-based activation motif-bearing adaptor DAP12. MAIR-I is also known as CD300a/ CMRF-35-like Ig-like molecule-8 (CLM-8)/leukocyte mono-Ig-like receptor 1 (LMIR1). MAIR-II is also known as CD300d/LMIR2/CLM-4/dendritic cell-derived Ig-like receptor 1 (DIgR1).