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CYCSA gene on chromosome 7p15.3 that encodes cytochrome c, an electron carrier protein, which transfers electrons from the heme group of the cytochrome c1 subunit of cytochrome reductase to the cytochrome oxidase complex, the final protein carrier in the mitochondrial electron-transport chain. CYCS plays a role in apoptosis by suppressing anti-apoptotic members or by activating pro-apoptotic members of the Bcl-2 family, leading to altered mitochondrial membrane permeability and the release of cytochrome c into the cytosol. Binding of cytochrome c to Apaf-1 activates caspase-9, which then accelerates apoptosis by activating other caspases.
Defects in CYCS cause thrombocytopaenia type 4.