In mammalian cells, once cytochrome c is released, it binds to the cytosolic protein Apaf-1 to facilitate the formation of an apoptosome. An early biochemical study suggests a two-to-one ratio of cytochrome c to apaf-1 for apoptosome formation. However, recent structural studies suggest the cytochrome c to apaf-1 ratio is one-to-one. It has also been shown that the nucleotide dATP as third component binds to apaf-1, however its exact role is still debated. The mammalian apoptosome had never been crystallized, but a human APAF-1/cytochrome-c apoptosome has been imaged at lower (2 nm) resolution by cryogenic transmission electron microscopy in 2002, revealing a heptameric wheel-like particle with 7-fold symmetry. Recently, a medium resolution (9.5 Ångström) structure of human apoptosome was also solved by cryo-electron microscopy, which allows unambiguous inference for positions of all the APAF-1 domains (CARD, NBARC and WD40) and cytochrome c. There is also now a crystal structure of the monomeric, inactive Apaf-1 subunit (PDB 3SFZ).