Trastuzumab is an anti-EGFR drug used in the treatment of HER2-positive breast cancer. EGFR is linked multiple signalling pathways involved in tumour growth and angiogenesis such as the Ras/Raf pathway and the PI3K/Akt pathways. These pathways ultimately lead to the activation of transcription factors such as Jun, Fos, and Myc, as well as cyclin D1, which stimulates cell growth and mitosis. Uncontrolled cell growth and mitosis leads to cancer. Trastuzumab acts as an anticancer drug by binding to the extracellular domain of the EGFR and preventing its activation by epidermal growth factor. This in turn inhibits downstream signalling and prevents tumour growth.