This is a family of bacterial 3TM holins. In Rhodobacter capsulatus the protein is expressed just overlapping and downstream of a putative N-acetylmuramidase lysozyme (an endolysin) thought to be responsible for lysing a phage particle, RcGTA - a gene-transfer agent. A holin would be necessary for such an endolysin to access the peptidoglycan. Gene-transfer agents are bacteriophage-like genetic elements with the sole known function of horizontal gene transfer, serving an important role in microbial evolution. In order to be released from the cell these require the combined action of an endolysin and this holin.