MDL-1 (Myeloid DAP12-associating lectin 1), also known as CLEC5A, is an approximately 40 kDa member of the C-type lectin family (1). Mature mouse MDL-1 is a glycosylated type 2 transmembrane protein that associates into a homodimer on the cell surface (2). It consists of a 165 amino acid (aa) extracellular domain (ECD) with one C-type lectin (CTL) domain and a juxtamembrane stalk region, a 21 aa transmembrane segment, and a 4 aa cytoplasmic domain (3). Within the ECD, mouse MDL-1 shares 67% and 81% aa sequence identity with human and rat MDL-1, respectively. The transmembrane segment contains a lysine residue that mediates interactions with the signaling protein DAP12 (3). Alternative splicing generates a short isoform of mouse MDL-1 that lacks 25 aa of the stalk region (3, 4). MDL-1 is expressed on monocytes, macrophages, and neutrophils (3, 4). Its expression is up-regulated on monocytes in rheumatoid arthritis (5). MDL-1 functions as a cell attachment receptor for all four serotypes of dengue virus as well as Japanese encephalitis virus, although the short isoform binds significantly more weakly (2, 6, 7). These interactions trigger DAP12 phosphorylation, the production of multiple inflammatory cytokines, vascular leakage, and disruption of the blood-brain barrier (6-8). This recombinant protein corresponds to the long isoform of mouse MDL-1.