Iron-sulfur clusters containing 4 atoms of iron and 4 atoms of sulfur (4Fe-4S clusters) are assembled in the cytosol on a heterotetrameric scaffold composed of NUBP2 and NUBP1 subunits (reviewed in Lill et al. 2012, Rouault et al. 2012, Sharma et al. 2010, Lill and Muhlenhoff 2006). The sources of iron and sulfur are uncertain but the process requires a sulfur-containing compound exported from mitochondria via ABCB7 (ABC7). Newly synthesized 4Fe-4S are transferred to apoproteins such as XPD and POLD1 via the CIA targeting complex, composed of NARFL, CIAO1, FAM96B, and MMS19.