Required for the assembly and stability of the mitochondrial ubiquinol-cytochrome c reductase complex (complex III (CIII) or cytochrome b-c1 complex), a multisubunit transmembrane complex that is part of the mitochondrial electron transport chain (ETC) which drives oxidative phosphorylation (By similarity). Mediates early complex III biogenesis (By similarity). Participates in regulating the levels of electron transport chain proteins, and therefore energy supply, in response to changes in energy demand (By similarity). Also involved in the first steps of cytochrome c oxidase complex (complex IV) assembly (PubMed:34969438). {ECO:0000250|UniProtKB:Q8C1Q6, ECO:0000269|PubMed:34969438}. This is the function of ENSG00000168273 (UQCC5, ubiquinol-cytochrome c reductase complex assembly factor 5).