HER2 is a receptor tyrosine kinase. In 30 percent of human breast cancers, HER2 is permanently activated, resulting in an unregulated cell division. Lapatinib, a drug used to treat breast cancer, inhibits HER2 receptor tyrosine kinase autophosphorylation (the process by which the receptor adds phosphates onto itself), thus reducing tumor growth by 50 percent. Besides autophosphorylation, which of the following steps would be inhibited by Lapatinib?
a. Signaling molecule binding, dimerization, and the downstream cellular response.
b. Dimerization, and the downstream cellular response.
c. The downstream cellular response.
d. Phosphatase activity, dimerization, and the downstream cellular response.

Lapatinib affects on HER2 receptor tyrosine kinase autophosphorylation and inhibit it's autophosphorylation happening on receptor's tail. Along with this, the receptor's downstream response will also inhibited because when there is no phosphorylation of tyrosine kinase receptor then it will not be activated and at the end signal will not be transduced downward to the transcription of particular gene.