The FISH method clearly demonstrated that all patients with the TEL/AML1 fusion had subpopulations of leukemic cells with deletion of the normal TEL allele, which is significant for understanding the progression of leukemia with t(12;21)..     ///    "Of the seven patients with the TEL/AML1 fusion, five exhibited normal karyotype, one was t(8;12)(q11;p13), i(21q) and the remaining one exhibited a near-triploid karyotype in conventional G-banding. "    ///    "On the other hand, no TEL/AML1 fusion positive patients were found among 37 adult ALL. "    ///    "We detected seven TEL/AML1 fusion positive patients (9.5%), all of whom were diagnosed as B-lineage ALL, among 74 childhood ALL. "