The human epidermal growth factor 2 – also known as HER2 – is a protein that tells human cells when to grow and when not to grow. Sometimes, a random genetic mutation or extra copies of the gene that encodes HER2 make these cells grow even when they shouldn’t. 

This type of cancer is called “HER2 positive.” Because it grows more quickly, it’s more aggressive than other cancers. Bladder cancer, gastrointestinal cancers, ovarian...