We determined whether the TEL-AML1 fusion gene can also be observed in continuous human leukemia cell lines with an early B-lineage phenotype.     ///    Occurrence of TEL-AML1 fusion resulting from (12;21) translocation in human early B-lineage leukemia cell lines..     ///    Twenty-nine such cell lines established from children (n = 13) or adults (n = 13) with early B-lineage ALL and five cell lines derived from chronic myeloid leukemia in blast crisis or B cell non-Hodgkin's lymphoma were investigated for the occurrence of the TEL-AML1 rearrangement by RT-PCR.     ///    "though neither reciprocal AML1-TEL, nor normal TEL, mRNA was detectable by RT-PCR in this cell line. "    ///    "While all 13 adult early B-lineage ALL cell lines and the five cell lines from other leukemias or lymphomas were negative, 1/13 pediatric cell lines (cell line REH) was found to be positive for TEL-AML1. "    ///    "These findings in REH provide a further example of, and possible cytogenetic mechanism for, the paradigm of TEL-AML1 fusion accompanied by deletion of the residual TEL allele. "    ///    "Using mainly reverse transcriptase-polymerase chain reaction (RT-PCR), the TEL-AML1 chimeric transcript has been observed in 22-27% of pediatric patients with acute lymphoblastic leukemia (ALL), in particular in the early B-lineage ALL subtype, making it the most common genetic lesion in these patients. "