Any large protein complex that contains Ecsit and NDUFAF1, is located in the mitochondrion, and is involved in the assembly of complex I of the oxidative phosphorylation system. In mammalian cells, three complexes of approximately 500, 600, and 850 kDa containing the 45 kDa isoform of Ecsit and NDUFAF1 have been observed. [ http://www.ncbi.nlm.nih.gov/pubmed/17344420 ]