ERBB2 and breast cancer: In addition, a study by Xu et al. suggested that molecular method such as mRNA may better serve on defining HER2-low cancer for the treatment decision needs due to its relatively broader dynamic range [68]; while, a recent study found neither IHC nor HER2 mRNA measured by qRT-PCR method would be optimal to quantify HER2-low expression, especially for HER2 1+ BC [69].