Understanding cancer biomarkers for diagnosis and prognosis leads to improved patient treatments and care. This thesis addresses the relevance of thymidine kinase 1 (TK1) as a cancer biomarker and the role of TK1 in cancer progression. Worldwide, cancer leads to more than 12 million deaths annually. In the United States alone, each year over 1.5 million cases will be diagnosed and over half a million persons will die. The most prevalent cancer types include skin, lung, breast, prostate, and colon. TK1 is known to be present in the serum of patients with multiple cancer types, including lung, breast, colon and prostate. In fact, it is shown to be detectable in cancer patients even before they manifest clinical symptoms. Additionally, the levels of TK1 increase progressively with increasing tumor grade; meaning that levels of TK1 can indicate tumor grade. Cellular proliferation markers such as p53 and Ki-67 have been compared to TK1 in cancer diagnosis and prognosis. TK1 has potential as both a prognostic and diagnostic biomarker in various cancer types including breast. Breast cancer is one of the most aggressive cancer types with 20-30% of diagnosed tumors becoming metastatic. Recent findings have identified additional involvement of TK1 downstream of cellular proliferation in cancer progression, including cellular invasion which is a part of cancer metastasis. These findings while efficacious, fail to identify the individual contribution of TK1 in downstream processes that aid in cancer progression. As mentioned previously, TK1 is upregulated in several different cancer types. We propose that there is an advantage to upregulated levels of TK1 in cancer progression and seek to explore its role specifically in cell invasion and survival. Based on our current understanding of TK1, we first wanted to review the history of TK1 and show the importance of understanding this crucial enzyme. Finally, we report our results from experiments exploring the influence of TK1 in vitro on breast cancer cell invasion and survival.