The S24, S26 LexA/signal peptidase superfamily contains LexA-related and type I signal peptidase families. The S24 LexA protein domains include: the lambda repressor CI/C2 family and related bacterial prophage repressor proteins; LexA (EC 18.104.22.168), the repressor of genes in the cellular SOS response to DNA damage; MucA and the related UmuD proteins, which are lesion-bypass DNA polymerases, induced in response to mitogenic DNA damage; RulA, a component of the rulAB locus that confers resistance to UV, and RuvA, which is a component of the RuvABC resolvasome that catalyzes the resolution of Holliday junctions that arise during genetic recombination and DNA repair. The S26 type I signal peptidase (SPase) family also includes mitochondrial inner membrane protease (IMP)-like members. SPases are essential membrane-bound proteases which function to cleave away the amino-terminal signal peptide from the translocated pre-protein, thus playing a crucial role in the transport of proteins across membranes in all living organisms. All members in this superfamily are unique serine proteases that carry out catalysis using a serine/lysine dyad instead of the prototypical serine/histidine/aspartic acid triad found in most serine proteases.