Actually not only cancer. That's also required for adaptive immunity. A human immune system needs roughly 100 million different antibodies to potentially bind e

Actually not only cancer. That's also required for adaptive immunity. A human immune system needs roughly 100 million different antibodies to potentially bind every pathogen. And there are only ~30K genes. What happens is that in B cells the DNA that encodes antibodies' binding regions actually mutate upon B cell maturation. Similar for T cells. The term is "V(D)J recombination".