Scheme 1. Chemical and Enzymatic Routes for the Production of Chito-Oligosaccharides (COSs): (A) Partial Acid Hydrolysis Degrades Chitin from Crustacean Shells or Squid Pens Into Mixtures of GlcN or GlcNAc Residues, paCOSs, And/or Fully Deacetylated Oligosaccharides with Various DPs Depending on the Acid Treatment Conditions (R = NH₂ or NHAc); (B) Endo-Chitinase Hydrolysis of Chitin Typically Generates a Range of COSs with Varying DPs (n = 1–9) Depending on the Enzyme and Substrate Used As Well As the Duration of the Treatment; (C) Endo-Chitosanase Hydrolysis of fully Deacetylated Chitosan, for example Chitosanase from Streptomyces griseus, Leads to the Formation of COSs of Different DPs (n = 2–6); and (D) Regioselective Deacetylation Using Site-Specific Chitin Deacetylases to Obtain Homogeneous Glycoform of Partially Deacetylated COSs.