Two-dimensional BN-PAGE analysis of assembly of complex IV. Mitochondrial translation products were pulse-labelled for 1 h, chased for 18 h and analysed on a two-dimensional BN-PAGE gel. Free COXI (S1) accumulated in patient fibroblasts relative to the fully assembled complex (S4). The migration of complexes V, III and IV in the one-dimensional BN-PAGE is indicated on the top of the figure. The position of individual complex IV subunits (COXI, COXII, COXIII and COXI dimer) is indicated on the right. S1, S3 and S4 are subassemblies of complex IV.