Members of this protein family are NifS, one of several related families of cysteine desulfurase involved in iron-sulfur (FeS) cluster biosynthesis. NifS is part of the NIF system, usually associated with other nif genes involved in nitrogenase expression and nitrogen fixation. The protein family is given a fairly broad interpretation here. It includes a clade nearly always found in extended nitrogen fixation genomic regions, plus a second clade more closely related to the first than to IscS and also part of NifS-like/NifU-like systems. This model does not extend to a more distantly clade found in the epsilon proteobacteria such as Helicobacter pylori, also named NifS in the literature, built instead in TIGR03403.