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The proof for the prenatal origin of childhood acute lymphoblastic leukemia (ALL) comes from the detection of concordant leukemia in monozygotic twins and the identification of translocation breakpoint genomic sequences at birth in a limited number of ALL patients with t(4;11) or t(12;21) chromosomal translocation.However, most patients with childhood ALL lack leukemia-specific fusion gene sequences.Therefore, we have used the rearranged immunoglobulin heavy chain (IgH) genes as a marker for the detection ofpreleukemic clones at birth.Guthrie card blood spots of 32 children with B-lineage ALL treated at our institution were available for this retrospective study.The ALL patients had a median age of 5 years and had median presenting white blood cell (WBC) counts of 10150/μl.In all patients a monoclonal IgH gene rearrangement was obtained from diagnostic bone marrow and sequenced.Clone-specific primers were designed using the specific D-N-J and N-D-N sequences.A two-stage polymerase chain reaction (PCR) using a semi-nested approach was developed to improve sensitivity and specificity of amplification.In all 32 patients, one leukemic cell could be detected in a background of 105 normal blood mononuclear cells.Nineteen of the 32 patients (59 %) had detectable IgH gene rearrangements at birth using the sensitive semi-nested PCR.Sequencing of the PCR products obtained from Guthrie card blood spots revealed the identical sequences identified from diagnostic leukemic cells.Interestingly, five of the six children (83%) with hyperdiploid ALL had detectable preleukemic clones at birth.Four of the five children (80%) with pro-B ALL, 13 of the 21 children (62%) with cALL and only two of the six children (33%) with pre-B ALL had preleukemic clones on their cards.We did not observe any differences in age at diagnosis or presenting WBC count between the 19 patients with preleukemic clones at birth and the 13 patients whose Guthrie cards were tested negative.Our results suggest that the majority of children with B-lineage ALL has preleukemic clones already at birth indicating a prenatal origin of leukemia.In addition, postnatal factors are important in leukemogenesis as well because of the long latency periods until clinical diagnosis of leukemia.