A variety of chromosomal translocations to the ALL-1 gene are regularly observed in acute leukemias and are thought to play a key role in the leukemogenic process. Chimeric proteins are encoded by the breakpoint regions of the derivative chromosomes have been proposed to be the relevant oncogenic agents. In addition, internal duplications of the ALL-1 gene have been observed in patients with specific acute myeloid leukemias. Thus, it has been hypothesized that oncogenic variants of the ALL-1 protein may be generated by both chimerization and self-fusion, but the critical structural features endowing the altered proteins with their oncogenic potential are still unknown. Here a novel structural alteration of the ALL-1 gene was observed in three patients presenting with acute T-cell leukemia (ALL) without chromosomal translocations or self-fusions of the ALL-1 gene. These unrelated patients carried an internal deletion in one of the two alleles of the ALL-1 gene that eliminated parts of introns 7 and 8, together with exon 8. The deletion was found in 3 of 74 ALL patients, but not in acute myeloid leukemias, follicular lymphomas, or peripheral blood leukocytes from healthy donors. One ALL patient showed the deletion at diagnosis but no longer at remission or at 9 months after remission. These findings support the hypothesis that the ALL-1 protein may be converted to an oncogenic variant, not only by chimerization or self-fusion, but also by deletion of sequences coded by exon 8. They further suggest that these three different types of structural alterations of the ALL-1 protein may each cause a distinct disease phenotype. Alternatively spliced mRNA species omitting exon 8 were observed in 14 of 24 ALL patients without detectable macroscopic alterations of the ALL-1 gene and also in peripheral blood leukocytes from healthy donors.