Haemolysin-type calcium-binding repeat Gram-negative bacteria produce a number of proteins that are secreted into the growth medium by a mechanism that does not require a cleaved N-terminal signal sequence. These proteins, while having different functions, seem to share two properties: they bind calcium and they contain a multiple tandem repeat of a nonapeptide [<cite idref="PUB00001195"/>]. The nonapeptide is found in a group of bacterial exported proteins that includes haemolysin, cyclolysin, leukotoxin and metallopeptidases belonging to MEROPS peptidase family M10 (clan MA(M)), subfamily 10B (serralysin). <p>It has been suggested that the internally repeated domain of haemolysin may be involved in Ca-mediated binding to erythrocytes. It has been shown that such a domain is involved in the binding of calcium ions in a parallel beta roll structure [<cite idref="PUB00001236"/>].</p>