This family consists of several bacterial NnrS like proteins. NnrS is a putative heme-Cu protein (NnrS) and a member of the short-chain dehydrogenase family. Expression of nnrS is dependent on the transcriptional regulator NnrR, which also regulates expression of genes required for the reduction of nitrite to nitrous oxide, including nirK and nor. NnrS is a haem- and copper-containing membrane protein. Genes encoding putative orthologues of NnrS are sometimes but not always found in bacteria encoding nitrite and/or nitric oxide reductase.