The primary type of ancillary module, which is common to most glycoside hydrolases, is the CBM (Linder and Teeri 1997; Boraston et al. 2004). Many CBMs serve to target the parent glycoside hydrolase to the substrate. The first CBMs to have been described were initially termed CBDs (cellulose-binding domains), owing to their substrate specificities and binding to crystalline types of cellulose. The CBDs were thus divided into “types” on the basis of amino acid sequence, in a manner similar to the GH families. Further work, however, revealed that some of the CBD types were not specific for crystalline cellulose (such as type 4) or to cellulose at all (some members of type 2 bound to cellulose, whereas others bound to xylan). Moreover, some protein modules were found to exhibit binding specificity to non-cellulosic polysaccharides. Today, the different CBMs are now divided into over 50 different families showing broad specificity patterns, sometimes within a given family and even by a given module. The CBMs exhibit various functions, including targeting of the parent enzyme to the undigested substrate, targeting of given modules to portions (conformations) of the sub­strate during deconstruction, and attachment of the parent enzyme to the microbial surface.