ERBB2 and breast cancer: Another consideration for patients with synchronous mCRC and breast cancer is to determine the HER2 (ERBB2) status of the mCRC, as approximately 3% of mCRC are HER2 positive, and 5% of KRAS and NRAS wild-type tumors.58 Similar to breast cancer, HER2 amplified mCRC can indicate more aggressive disease.