The C-termini of the glucosyltransferases (Gtfs) of oral streptococci are responsible for glucan binding. These glucan-binding domains (GBDs) are composed of a series of repeated sequences that have been classified into four different classes (A-D) by virtue of sequence similarity and which, by inference, have been suggested to be of functional importance. In contrast, we propose that repeat sequences evolve in response to selection for an increase in the number of copies of a particular domain through multiple duplication events occurring at different times. According to this hypothesis, repeats should possess various degrees of similarity, especially if only key residues are of functional importance. Analysis of the GBDs of the Gtfs indicated that a common fundamental repeat, designated the “YG” repeat, could be discerned within the “A“, “B”, “C”, and “D” repeats. Similar elements were also conserved in the ligand-binding repeats of the Clostridium difficile toxins and the lysins and the PspA protein of Streptococcus pneumoniae, suggesting that similar selective pressures had also been imposed on these sequences. Analysis of the “YG” repeats present in the GtfJ and GtfK of Streptococcus salivarius indicated that some of the “YG” repeats in the GBDs of these proteins had arisen as a result of duplication events involving a series of three sequential “YG” repeats.