Iron-sulfur clusters containing 4Fe-4S are assembled from 2Fe-2S clusters on ISCA1:ISCA2 heterodimers (Banci et al. 2014, Brancaccio et al. 2014, inferred from Saccharomyces cerevisiae in Mühlenhoff et al. 2011). GLRX5:2Fe-2S can donate 2Fe-2S clusters to ISCA1:ISCA2 in vitro (Banci et al. 2014, Brancaccio et al. 2014). It is unclear if other proteins also donate 2Fe-2S clusters. Two conserved C-terminal cysteines of ISCA1:ISCA2 heterodimers extract [2Fe-2S] clusters from GLRX5, forming a ISCA1:ISCA2:GLRX5 intermediate containing two 2Fe-2S clusters (Brancaccio et al. 2017). The physiological electron donor required to convert the two 2Fe-2S clusters bound to the intermediate into a 4Fe-4S cluster is not yet characterized. ISCA1, ISCA2, and IBA57 are required for formation of holoenzymes such as aconitase that contain 4Fe-4S clusters (Sheftel et al. 2012). HSCB (HSC20), the homolog of yeast JAC1, interacts with HSPA9 and appears to facilitate the reaction (Uhrigshardt et al. 2010).