Molecular Characterization of the Human trk Oncogene

The human genome contains loci that, when mutated in their coding or regulatory sequences, may lead to the induction of phenotypic changes characteristic of neoplastic cells (Varmus 1984; Barbacid 1985). These loci are designated proto-oncogenes, whereas their mutated alleles are known as oncogenes. So far, more than 20 human oncogenes have been identified (Table 1). Some of them are present in a significant fraction of human tumors independently of their clinical manifestations (e.g., ras gene family). Some are specifically activated in certain forms of human cancer (e.g., abl, myc gene families), whereas others are occasionally found in isolated tumors (e.g., trk). Finally, a few of these loci have been identified as a result of their sporadic activation during in vitro manipulation of human DNA (e.g., mas, ret).