From the MTT results, we found that human breast cancer cell lines containing EGFR gene amplification with high expression of EGFR (MDA-MB-468) showed an IC50 value of 3.31 μM, whereas the other cell lines with EGFR gene amplification but with lower EGFR expression, such as HCC1937, showed an IC50 of 9.02 μM. The breast cancer cells that overexpress HER2 exhibited a similar response to 5a. The gene discussed is EGFR; the disease is breast cancer.