During the last two decades since the first oncogene product called v-Src was identified as a constitutively activated mutant of a normal mitogenic gene (proto-oncogene) encoding a Tyr kinase called c-Src in early 1980s, it was firmly established that the malignant transformation of normal cells is caused by a combination of the following genetic events: overexpression of a protooncogene or constitutive activation of its gene product (mitogenic signal transducer), and deletion of a tumor-suppressor gene (also called anti-oncogene or anti-mitogenic gene) or dysfunction of its gene product (anti-mitogenic signal transducer).
