Glycoside hydrolase family 57 glycogen branching enzymes (GH57GBE) catalyze the formation of an α-1,6 glycosidic bond between α-1,4 linked glucooliogosaccharides. As an atypical family, a limited number of GH57GBEs have been biochemically characterized so far. This study aimed at acquiring a better understanding of the GH57GBE family by a systematic sequence-based bioinformatics analysis of almost 2,500 gene sequences and determining the branching activity of several native and mutant GH57GBEs. A correlation was found between a very low or even no branching activity with the absence of a flexible loop, a tyrosine at the loop tip, and two β-sheets.