This family contains a set of membrane proteins, typically 33 amino acids long. The family has no known function, but the protein is found in the operon CydAB in E. coli. Members have a consensus motif (MWYFXW) which is rich in aromatic residues. The protein forms a single membrane-spanning helix. This family seems to be restricted to Proteobacteria [1].

1: Characterization of the tol-pal and cyd region of Escherichia coli K-12: transcript analysis and identification of two new proteins encoded by the cyd operon. Muller MM, Webster RE; J Bacteriol 1997;179:2077-2080. PMID:9068659