Mitochondrial Complex I (MCI) assembly is a multistep process involving the number of assembly of intermediate modules that assemble to form the mature complex. MCI assembly factors (CIAFs) assist in the assembly and stability of the modules and the insertion of cofactors. MCI is assembled from six modules: the matrix arms modules N and Q and the membrane arm modules: ND1, ND2, ND4 and ND5 which are assembled separately. First, ND2- and the ND4-modules assemble, followed by the Q/ND1-, the ND5-modules. Lastly the N-module joins and the CIAFs dissociate from the holoenzyme. (Adapted from PMID:32320651, PMID:30030361, FBrf0244962 and FBrf0235957.)