However, it is important to recall that the homozygous deletion of many well-established breast cancer drug targets, such as human epidermal growth factor receptor (HER2), mTOR, and the catalytic domain of PI3K, are also embryonic lethal [38-41], yet this fact did not preclude or even delay the successful clinical development of therapeutics selective for these targets. The gene discussed is ERBB2; the disease is breast cancer.