src expression in small-cell lung carcinoma and other neuroendocrine malignancies. The proto-oncogene c-src codes for two tyrosine kinases, pp60c-src and pp60c-srcN. The latter protein appears to be exclusively expressed in neurons and neuronally differentiated tumors. In cell lines derived from neuroblastoma and small-cell lung carcinoma, src expression correlates positively with neuroendocrine differentiation. However, pp60c-srcN is expressed only in highly differentiated neuroblastomas. Although c-src expression in neuroendocrine tumors probably reflects and is the result of the differentiation stage at which the tumors have been arrested, high c-src expression and kinase activities in non-neuroectodermal tumors, e.g., colon carcinoma, breast carcinoma, might instead be a part of the malignant phenotype and contribute to the development of these tumors. 