This family of proteins are found in a range of bacteria. It has been shown that this domain can act as an L,D-transpeptidase that gives rise to an alternative pathway for peptidoglycan cross-linking. This gives bacteria resistance to beta-lactam antibiotics that inhibit PBPs which usually carry out the cross-linking reaction. The conserved region contains a conserved histidine and cysteine, with the cysteine thought to be an active site residue. Several members of this family contain peptidoglycan binding domains. The molecular structure of YkuD protein shows this domain has a novel tertiary fold consisting of a beta-sandwich with two mixed sheets, one containing five strands and the other, six strands. The two beta-sheets form a cradle capped by an alpha-helix. This family was formerly called the ErfK/YbiS/YcfS/YnhG family, but is now named after the first protein of known structure.