Aberrant expression of transcription factor oncogenes such as HOX11, HOX11L2, 
TAL1/SCL, LYL1, LMO1, and LMO2 can be detected in lymphoblasts from up to 80% of 
patients with acute T-cell lymphoblastic leukemia (T-ALL). Transcriptional 
activation of these oncogenes in leukemic cells typically results from 
chromosomal rearrangements that place them next to highly active cis-acting 
transcriptional regulatory elements. However, biallelic activation of TAL1 in 
some T-ALL cases has been previously proposed. We have used allele-specific mRNA 
analysis to show that trans-acting mechanisms leading to biallelic 
overexpression of TAL1 are involved in 10 (42%) of 24 TAL1+ informative T-ALL 
cases, 2 (17%) of 12 HOX11+ informative cases, and 7 (64%) of 11 LMO2+ 
informative cases. We propose that aberrant expression of oncogenic 
transcription factors in a significant fraction of T-ALLs may result from loss 
of the upstream transcriptional mechanisms that normally down-regulate the 
expression of these oncogenes during T-cell development.