Due to a few difference between HER3 and other members of the human epidermal growth factor receptor family, such as HER2 developing radiotracer for HER3 imaging is much more challenging because: firstly; even in the case of HER3 overexpression in cancer, the number of HER3 does not exceed 50000 receptors/cell and, secondly; normal human tissues, such as the respiratory, urinary, gastrointestinal, and reproductive tract and even skin physiologically express HER3 receptor. The gene discussed is ERBB3; the disease is cancer.