Working out the order in which the individual components in a signaling pathway act is an essential step in defining the pathway. imagine that two protein kinases, pk1 and pk2, act sequentially in a kinase cascade. when either kinase is completely inactivated, cells do not respond to the normal extracellular signal. by contrast, cells containing a mutant form of pk1 that is permanently active respond even in the absence of an extracellular signal. doublymutant cells that contain inactivated pk2 and permanently active pk1 respond in the absence of a signal. in the normal kinase cascade, does pk1 activate pk2 or does pk2 activate pk1?