Adult T-cell leukemia (ATL) is usually defined as a malignant disease of T cells infected by human T-lymphotropic virus type I (HTLV-I). In the present study, we describe a 49-year-old woman with an acute type ATL, whose leukemic cells do not contain the HTLV-I genome. Laboratory tests revealed an increase in abnormal lymphocytes with convoluted nuclei, elevated serum lactate dehydrogenase levels, increased thymidine kinase activity and soluble interleukin-2 receptor-α levels. Serum examination demonstrated positive anti-HTLV-I antibody, but Southern blot analysis using the whole HTLV-I genome as a probe did not detect any integration of the viral genome. In contrast, PCR detected the HTLV-I pX region in the same DNA samples as used for Southern blot analysis. These findings suggest two possibilities. One possibility is that ATL in this patient is generated by other pathogens than HTLV-I virus. She is also an HTLV-I carrier. The other possibility is that her leukemic T cell clone derived its malignant phenotype from HTLV-I infection, and once this malignant phenotype was obtained, partial deletions of viral genome repeated until the whole viral genome was deleted. Although there is no direct evidence, the former possibility is more likely in the present case.

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