Sialic acid-binding immunoglobulin-like lectins (Siglecs)1 are transmembrane sialic acid-binding proteins of the immunoglobulin (Ig) superfamily characterized by the presence of an N-terminal V-set Ig-like domain and variable numbers of C2 set domains (1). The first Siglecs to be characterized were Siglec-1, Siglec-2, Siglec-3 and myelin-associated Siglec-4 which share ∼25–30% sequence identity within the extracellular regions. Recent studies have uncovered the existence of a cluster of genes on human chromosome 19q13.3-4 that encode novel Siglecs highly related to CD33. This CD33-related subgroup includes Siglecs-3, -5, -6, -7, and -8, each of which share ∼50–70% sequence identity. These differences in expression and ligand binding suggest that each Siglec mediates a specific, nonredundant function in hemopoietic cell biology. Siglec-9 is ∼80% identical in amino acid sequence to Siglec-7, suggesting that the genes encoding these two proteins arose relatively recently by gene duplication. similar to Siglec-7, Siglec-9 recognized sialic acid in either the α2,3- or α2,6-glycosidic linkage to galactose. Using a specific mAb, Siglec-9 was found to be expressed at high or intermediate levels by monocytes, neutrophils, and a minor population of CD16⁺, CD56⁻ cells.