Previously we have described the derivation of three distinct classes of leukemic cell clones from a single in vivo-passaged myelomonocytic leukemia, WEHI-274, that arose in a mouse infected with the Abelson leukemia virus/Moloney leukemia virus complex (K. B. Leslie and J. W. Schrader, Mol. Cell. Biol. 9:2414-2423, 1989). The three classes of cell clones were characterized by distinct patterns of growth in vitro, the production of cytokines, and the presence of cytokine gene rearrangements. However, all three classes of WEHI-274 clones bore a common rearrangement of the c-myb gene, suggesting that all were derived from the one ancestral cell and that at least three distinct and independent autostimulatory events were involved in the progression of a single myeloid leukemic disease. In this article, we demonstrate that the autocrine growth factor production by the WEHI-274 leukemic clones resulted from cytokine gene activations mediated by the insertion of an intracisternal A-type particle (IAP) sequence 5' to the interleukin-3 (IL-3) gene, in the case of the class I clone, or 5' to the gene for granulocyte-macrophage colony-stimulating factor (GM-CSF), in the case of the class II clones. IAPs are defective murine retroviruses encoded by endogenous genetic elements which may undergo transpositions and act as endogenous mutagens. The functional IL-3 and GM-CSF mRNAs were generated by mechanisms in which the splice donor apparatus of the IAP sequence has been used in IAP gag-to-IL-3 or -GM-CSF splicing events.