In a multicellular organism such as a mammal, loss-of-function mutations in many genes can contribute to the develpment of cancer. These genes are...

In a multicellular organism such as a mammal, loss-of-function mutations in many genes can contribute to the develpment of cancer.  These genes are therefore called tumor suppressors.  In their absence, the cell fails to stop progression through the cell cycle under conditions in which normal cells would arrest, paving the way for tumorigenesis.  Indicate whether each of the following proteins is (Y) or is not (N) expected to be the product of a tumor suppressor gene based on its function in the cell cycle.  

Rb

Myc

p53

p21