E. coli single-stranded DNA binding protein, SSB, is a protein that binds single-stranded DNA (ssDNA) present in the cell and prevents it from forming secondary structures. It is composed of four identical monomers each consisting of an OB domain and a C-terminal tail. The OB domain is responsible for binding to ssDNA as it contains the tryptophan and phenylalanine residues that interact with the nucleotide bases of ssDNA through base-stacking interactions. The C-terminal tail is responsible for binding other proteins. To date, 15 proteins involved in replication, repair, and recombination have been shown to bind SSB. Most of these binding partners have been shown to interact with the C-terminal tail of SSB. The binding sites of the remaining binding partners have not been determined. Recently our lab has determined that UmuD2, a protein involved in the cell’s SOS response to DNA damage, also binds SSB. Furthermore, we have determined that unlike other proteins, UmuD2 does not bind the C-terminal tail; instead, it binds the OB domain. We have determined that SSB inhibits cleavage of the UmuD N-terminal arms, a process important for regulating SOS mutagenesis. Research supported by NSF MCB-0845033.