In solid tumors, the cancer cells are in the increased state of hypoxia and interstitial fluid pressure due to the broken-down of homeostatic regulation [24]. to EGFR, not to the seven unrelated proteins as negative controls. They also bound to the three different human cancer cell lines, but not to the two cell lines as negative controls. They inhibited cell proliferation, migration and invasion and increased apoptosis of these three cancer cell lines. Two of them were tested for their anti-tumor effect in vivo and showed the anti-tumor activity in a mouse xenograft model for human lung cancer. Immunohistochemical staining of xenograft tumors also showed that their anti-tumor effects were associated with the inhibition of cancer cell proliferation and the promotion of cancer cell apoptosis. == Conclusions == This study clearly demonstrated that the anti-EGFR sdAbs could inhibit cancer cell growth in vitro and tumor growth in vivo. They could be potential therapeutics for the treatment of different TCS 359 human cancers. Keywords:EGFR, Cancer, Antibody, Single-domain antibody, Antibody phage library == Background == Epidermal growth factor receptor (EGFR) family comprises the four homologous members: EGFR or HER1, also known as ERBB1, HER2 or ERBB2, HER3 or ERBB3 and HER4 or ERBB4 [1]. Each member consists of an extracellular domain (ECD), a transmembrane domain and an intracellular domain (ICD) [2]. EGFR contains an extracellular domain (EGFR-ECD) of 620 amino acids, a transmembrane domain of 23 amino acids and an intracellular domain (EGFR-ICD) of 501 amino acids. EGFR-ECD comprises four domains, and the domains I and III include EGF binding sites. The EGFR-ICD includes a tyrosine kinase site. After EGF binds to EGFR, EGFR dimmer is formed and induces intracellular tyrosine phosphorylation, leading to the activation of the EGF/EGFR signaling pathway [3]. Studies indicate that the EGF/EGFR signaling pathway interacts with EGFR-ERK, EGFR-STAT3 and EGFR-mTOR signaling pathways [4]. These pathways are associated with tumor cell proliferation, vitality, migration and invasion. They are also associated with the angiogenesis in the tumor microenvironment, tumorigenesis and tumor progression and metastasis [5,6]. According to the previous reports, TCS 359 EGFR and HER2 are expressed mostly at a low level in all normal cells, while HER3 and HER4 are expressed at a moderate to high level in all normal cells. EGFR over-expression in cancer cells is closely associated with poor clinical prognosis, reduced survival rate and more aggressive phenotype [7]. EGFR over-expression was shown in several solid tumors, such as lung, breast and prostate cancers [810]. Therefore, EGFR may serve as an attractive target for cancer therapy of different cancers [11]. Monoclonal antibodies (mAbs) have become powerful therapeutics for cancer therapy. Trastuzumab is an anti-EGFR mAb approved for the treatment of breast cancer. Cetuximab is another mAb targeting EGFR and was approved for the treatment of metastatic colorectal cancer, metastatic non-small cell lung cancer and head and neck cancer. However, mAbs used for cancer therapy have many disadvantages. Mouse-derived mAbs can result in rapid antibody clearance, loss of efficacy or hypersensitivity reactions in cancer patients [12]. Along with the development of TCS 359 genetic engineering, chimeric antibodies or humanized antibodies were developed and reduced only some of their antigenicity [13]. MAbs are difficult to penetrate into solid tumors due to their large molecular weight and expensive to be manufactured with mammalian cells [14,15]. Thus, NCR3 small size antibodies are needed to replace the conventional mouse-derived mAbs for cancer therapy. Single-chain variable fragment (scFv) is a small-size antibody of about 40 kDa, consisting of only the variable domains of the.