classical cell line (70%) with well-defined neuroendocrine features, a low colony-forming efficiency, and highly responsive to chemo- and radiotherapy;

variant cell line (30%), with loss of neuroendocrine features, a high colony-forming efficiency, poorly responsive to chemo- and radiotherapy, and morphologically resembling the large cell undifferentiated large cancer type. This type is associated with overexpression of the c-myc oncogene.

Evolution of small cell lung cancer from classical to variant type may occur as a natural event, be induced by therapy or as a result of genetic developments and interaction between myc-oncogenes and Harvey-ras oncogenes.