Cytokines can have both negative and positive effects on cells undergoing carcinogenesis. The promotion and progression phases of carcinogenesis may be affected by autocrine loops involving cytokines with growth factor activities such as IL-1, IL-2, low molecular weight B cell growth factor, TNF, IL-3, GM-CSF, M-CSF and IL-9. Aberrations in cytokine receptors such as the truncated EGF receptor present in v-erB promotes the growth of neoplastic cells. Aberrant signaling mechanisms, as found with spleen focus-forming virus, which mimics the ligand that activates the erythropoietin receptor, can also contribute to proliferation of preneoplastic and neoplastic cells. In contrast, cytokines such as interferons, LIF, TGF-beta, TNF and leukoregulin, with antiproliferative or differentiating activities, are sometimes capable of inhibiting carcinogenesis. Transfection of tumor cells with cytokine genes, such as IL-2, IL-4 and TNF, can cause suppression of in vivo tumor cell growth by mobilizing host immune and inflammatory cell responses. Thus cytokines and their receptors may play a direct role in early stages of tumor cell development and growth.