The LexA repressor or LexA (Locus for X-ray sensitivity A)[1] is a transcriptional repressor (EC 3.4.21.88) that represses SOS response genes coding primarily for error-prone DNA polymerases, DNA repair enzymes and cell division inhibitors.

The DNA-binding domain is a variant form of the helix-turn-helix DNA binding motif,[4] and is usually located at the N-terminus of the protein.

Antibiotics such as ciprofloxacin are able to prevent the action of these molecules by attaching themselves to the gyrate–DNA complex, leading to replication fork stall and the induction of the SOS response.

The expression of error-prone polymerases under the SOS response increases the basal mutation rate of bacteria.

In addition, ciprofloxacin has also been shown to induce via the SOS response dissemination of virulence factors[5] and antibiotic resistance determinants,[6] as well as the activation of integron integrases,[7] potentially increasing the likelihood of acquisition and dissemination of antibiotic resistance by bacteria.