Many mutations in receptor kinases that lead to cancer allow the dimerization and activation of the receptor, even in the absence of signaling molecule. An example of this is a mutant form of the EGF receptor kinase called Her2/neu. An antibody that prevents dimerization of Her2/neu receptor kinases is being tested for its effectiveness in stopping cancer. At which stage would this drug work?

a. It would prevent the receptor from binding to the signal.
b. It would prevent the termination of the signal.
c. It would prevent the receptor from becoming activated.
d. It would prevent the signaling cell from producing the signal.

Signal transduction can be defined as the process by which the stimulus or message from the exterior of the cell is transferred to the interior of the cell so that the cell can develop a response according to the demand of the stimulus.
It takes place is the following manner:

Signalling molecules carrying the impulse binds to the complementary receptor molecules (Her2/neu) present on the surface of the cell membrane.
This binding causes the dimerization and activation of the receptor kinases (by auto-phosphorylation).
The activated receptor transmits a signalling cascade inside the cell through secondary messengers or enzymes or other protein activation that generates a response.

Mutation in the Her2/neu receptor causes its automatic dimerization and unstoppable activation.
This keeps the signalling pathway downstream of it to remain constitutively activate, contributing to cancer development.
Antibody preventing dimerization of the receptor would prevent the the auto-phosphorylation of the receptor and hence its activation.
This would also prevent the uncontrolled activation of the signalling pathway downstream of the Her2/neu receptor.