Though they are very similar to ethers, they are not ethers. They're sulfides.

Coupling of drug and bioactive molecule with chemical reactive groups such as amine, thiol, carboxylate, hydroxyl, aldehyde and ketone, active hydrogen, photo-chemical and cycloaddition reactions, zero-length cross-linking, homobifunctional, heterobiofunctional, or multifunctional cross-linking chemistries, dendrimer and dendrons, and cleavable reagent systems.
Bioconjugate chemistry may occur through the C- and N-terminals of each polypeptide chain, the carboxylate groups of aspartic and glutamic acids, the ϵ-amine of lysine, the guanidino group of arginine, the sulfhydryl group of cysteine, the phenolate ring of tyrosine, the indol ring of tryptophan, the thioether of methionine, and the imidazole ring of histidine. I got this from the website of Creative Peptides (creative-peptides.com)