TEL/AML1 fusion gene (derived from t(12;21)(p13;q22) translocation) became recently one of the most important genetic aberrations in children with ALL.     ///    TEL/AML1 act presumably as dominant inhibitors of the second AML1 allele and thus they block transcription of genes dependent on CBF factor.     ///    Childhood ALL with TEL/AML1 hybrid gene is very frequent (approximately 22% of overall childhood ALL in the Czech Republic) and patients with this fusion form relatively homogenous group. 