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The thymus gland, as a primary lymphoid organ, plays a crucial role in the normal development and appropriate functioning of the immune system, contributing to the maturation and differentiation of T lymphocytes. The first biologically active extract isolated from the thymus was named thymosin fraction V (TFV). The fractionation of TFV led to the isolation of a series of immunoreactive polypeptides, termed thymosins, the most important and well-studied of which are thymosin a1 (Ta1) and its precursor molecule, prothymosin a (proTa). Although the precise molecular mechanism of action of these polypeptides has not yet been fully elucidated, it is thought that they have a potential use in medical practice, both as cancer biomarkers and as immunotherapeutic agents for the treatment of cancer patients. ProTa is ubiquitously expressed in all tissues and plays a dual role; intracellularly, it participates in the regulation of the cell cycle, while extracellularly it acts as an immunomodulator. This review focuses on proTa and the data accumulated to date with respect to its role in cancer, and specifically its potential clinical utility, both as a biomarker for cancer prognosis and monitoring, and as an immunotherapeutic tool for the activation of immune responses against tumor cells. Based on the results derived from clinical trials of Ta1 in patients with cancer, the eventual use of proTa in similar studies in humans is discussed, aiming at the enhancement of the functions of the immune system and, ultimately, at cancer regression. [ABSTRACT FROM AUTHOR]