SIGLEC9 (sialic acid binding Ig-like lectin 9)
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Siglec-9 a sialic acid binding protein found on cells of the innate immune system, and may be involved in down-regulating innate immune responses. The homologous molecule found on the same cell types and chimpanzees and gorillas and appears to strongly prefer to bind to the non-human sialic acid Neu5Gc. This is in contrast to the human molecule, which prefers to binding the human sialic acid Neu5Ac. Thus the ancestral condition of the molecule was likely to prefer the sialic acid that was lost during hominid evolution. It is reasonable to speculate that the human molecule had to undergo adjustments to accommodate to bind Neu5Ac ligand. Indeed, there are several amino acid changes in the binding pocket off human Siglec-9 that a potentially involved in this evolution adjustment. It is unclear whether the evolutionary adjustment of binding is complete, and whether or not this has any consequences for modern human immunity or disease.