Mammalian cell surfaces are coated in specific sugar structures, many of which function as antigens and are involved in cellular recognition. Important examples are the oligosaccharide A, B, and H antigens present on red blood cells that differentiate the A, B and O blood types. Enzymatic cleavage of the GalNAc and Gal residues from the cell surface would allow conversion of A and B red blood cells, respectively, to O type. Since Type O blood can be universally donated to patients with the same Rh factor, access to efficient enzymes would greatly broaden and simplify blood supply. We have sought such enzymes in metagenomic libraries derived from the human gut microbiome...