The 2 most important initiator caspases are caspase8 and caspase9. These are associated with the 2 major pathways for initiating apoptosis. Caspase 8 is involved in receptor mediated apoptosis while caspase 9 is involved with the mitocondrial pathway. Mice deficient for either of these caspases usually die before birth and always within 3 days of birth. The mice show distinct defects for each caspase, some of which include brain deformities, heart malformations and blood overproduction.
Caspases are ubiquitously expressed, therefore every cell is poised for rapid self destruction.
The Bcl2 family of proteins are the central regulators of the mitochondrial pathway. Bcl2 is an inhibitor of apoptosis. Apaf is an activator of the initiator caspase9. It is thought that Bcl2 may somehow monitor damage in these compartments. Bax is a protein,related to Bcl2, which inhibits Bcl2 thereby promoting apoptosis.