The human thrombopoietin gene is located on chromosome 3q26.33-q27, but is not transcriptionally activated in leukemia cells with 3q21 and 3q26 abnormalities (3q21q26 syndrome).

We previously demonstrated that the EVI-1 gene was transcriptionally activated in the 3q21q26 syndrome and chromosomal breakpoints at 3q26 were clustered within 400 Kb of the EVI-1 gene. Since thrombocytosis is often observed in the 3q21q26 syndrome, we first mapped the thrombopoietin (TPO) gene and then we examined for transcriptional activation and chromosomal rearrangement of the TPO gene in four cases of the 3q21q26 syndrome. The TPO gene was assigned to chromosome 3q26.33-q27 by fluorescence in situ hybridization analysis. Although the TPO gene was mapped to the same locus as the EVI-1 gene, the distance between the TPO gene and the EVI-1 gene at 3q26 was more than 600 Kb and no gross chromosomal rearrangements of the TPO gene were detected by Southern blot analysis and pulsed field gel electrophoresis (PFGE) analysis. TPO transcripts were not detected in these leukemia cells by Northern blot analysis. These results indicate that activation of the TPO gene is not the main cause of thrombocytosis in the 3q21q26 syndrome.