Complex I is the largest enzyme of the respiratory chain, a fundamental metabolic pathway operating to supply the cell with energy. Dysfunction of complex I causes numerous neuromuscular and neurodegenerative diseases in humans. Combining biochemical and structural evidence the Zickermann group now succeeded in shedding light on the essential role of accessory subunit NB4M (NDUFA6/LYRM6) for complex I function. The results are published in the current early edition of the journal PNAS (Angerer et al., doi: 10.1073/pnas.1322438111).