sentences:  Caspase-3 and -7 are effector proteases whereas caspase-8 is an initiator caspase that sits more upstream in the apoptotic pathway.These results demonstrate survivin's capability to bind with particular caspases "in vitro", but may not necessarily translate over to actual physiological conditions.Later, a 2001 study confirmed that human survivin tightly binds caspase-3 and -7 when expressed in "E. coli".