These proteins are predicted transmembrane proteins with probably four transmembrane spans. The 1.E.40 is represented by the mycobacterial 4 phage holin, but it also contains many cyanobacterial. proteobacterial and firmicute proteins. Holins are encoded within the genomes of Gram-positive and Gram-negative bacteria as well as in those of the bacteriophage of these organisms. The primary function of holins appears to be transport of murein hydrolases across the cytoplasmic membrane to the cell wall where these enzymes hydrolyze the cell wall polymer as a prelude to cell lysis. When chromosomally encoded the enzymes are therefore autolysins. Holins may also facilitate leakage of electrolytes and nutrients from the cell cytoplasm, thereby promoting cell death. Some may catalyze export of nucleases.