History
The first described candidate Siglec was Sialoadhesin (Siglec-1/CD169) a lectin-like adhesion protein on macrophages. Parallel studies by Ajit Varki and colleagues on the previously cloned CD22 (a B cell surface protein involved in adhesion and activation) showed direct evidence for sialic acid recognition. The subsequent cloning of Sialoadhesin by Crocker revealed homology to CD22 (Siglec-2), CD33 (Siglec-3) and myelin-associated glycoprotein (MAG/Siglec-4), leading to the proposal for a family of "Sialoadhesins". Varki then suggested the term Siglec as a better alternative and as a subset of I-type (Ig-type) lectins. This nomenclature was agreed upon and has been adopted by almost all investigators working on these molecules (by convention, Siglecs are always capitalised.) Several additional Siglecs (Siglecs 5–12) have been identified in humans that are highly similar in structure to CD33 and so are collectively referred to as "CD33-related Siglecs". Further Siglecs have been identified including Siglec-14 and Siglec-15. Some Siglecs are absolutely conserved between mice and humans including Sialoadhesin, CD22, MAG and Siglec-15. Others such as Siglec-5 and Siglec-9 have homologues in mice and rats (Siglec-F and Siglec-E respectively in both). Humans have a higher number of Siglecs than mice and so the numbering system was based on the human proteins.
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