We hypothesize that coordination between the two DNA parental sets in somatic cells is essential for the stability of the diploid genome, and that its disruption is associated with the many alterations observed in the various cancerous phenotypes. As coordination between two allelic counterparts is well exemplified by synchrony in replication timing, we examined, in blood cells of patients suffering from various hematologic malignancies, replication patterns of five loci. These loci were three cancer-implicated genes (TP53, AML1, and RB1) and two nontranscribed sequences engaged in chromosome segregation. All five loci normally display synchrony in allelic replication timing. In addition, in order to exemplify an asynchronous mode of allelic replication, we followed the replication of allelic counterparts of an imprinted gene (SNRPN), which is distinguished by its asynchronous mode of allelic replication (allele-specific replication). Allelic replication patterns were studied by fluorescence in situ hybridization (FISH), which has been shown to distinguish between nonreplicated and replicated regions of the genome in interphase cells, based on the structure of the specific hybridization signals that are being detected. Using the FISH replication assay we observed, for all loci which normally exhibit synchrony in allelic replication, loss of synchrony when present in blood cells of patients with hematologic malignancies. The loss of synchrony in allelic replication in patients' cells was accompanied by aneuploidy (chromosome losses and gains), the hallmark of cancer. We were able to reinstate the normal pattern of replication in the patients' cells by introducing an inhibitor of DNA methylation. It thus appears loss of allelic coordination is an epigenetic alteration characterizing cancer, which is easily identified by simple cytogenetic means and has a potential use in both cancer investigation and detection.