Proto-oncogenes and germ-cell differentiation.

Oncogenes are identified functionally by their ability to induce neoplastic transformation of susceptible cells. The first oncogenes to be characterized were isolated from acutely transforming retroviruses. Subsequently, it was determined that the retroviral oncogenes were formed from normal, progenitor genes. These cellular homologs of the viral oncogenes are termed proto-oncogenes. The derivation of oncogenes from proto-oncogenes is the consequence of mutations that remove regulatory constraints from the proto-oncogene. The ability of oncogenes to induce transformation implies that proto-oncogenes may function in growth and differentiation pathways in normal cells. Although many proto-oncogenes have been defined, the normal physiological function of most is not known. Studies of proto-oncogene expression during normal gametogenesis have determined that some genes are expressed in a stage-specific manner. The use of germ cells to provide homogeneous and defined normal cell populations facilitates identifying the roles proto-oncogenes have in regulating cell growth and differentiation.