Recently, a new recurrent t(12;21)(pl3;q22) has been identified in a B-cell lineage childhood acute lymphoblastic leukemia (ALL). The translocation results in a fusion of two known genes, ETV6/TEL (12p13) and AML1 (21q22), previously shown to be involved in the pathogenesis of myeloid disorders. We report results of cytogenetic fluorescence in situ hybridization and molecular studies of a B-cell childhood common ALL with a cryptic 12;21 translocation. Aberrations identified in this case involve both chromosomes 12 and include not only the ETV6-AML1 gene fusion and two different microdeletions of ETV6 but also the hemizygous loss of CDKN1B, D12S119, and KRAS2 loci and a putative rearrangement of the second CDKN1B allele as a result of an inv(12)(p13q24). Moreover, it was shown that the AML1-ETV6 reciprocal chimeric transcript was not present in the malignant cells, and hence may not play a major role in leukemogenesis. In addition, the putative loss of wild-type function of CDKN1B and ETV6 could indicate a synergistic effect of both genes in the pathogenesis of this leukemia case.