Genetic instability is characteristic of cancer cells, both at the chromosomal level (e.g., aneuploidy, aneusomy, translocations), and at the sequence level (e.g., microsatellite instability). Colorectal cancers (CRCs) can be divided into two groups: tumors of the proximal colon, where microsatellite instability is frequent and chromosomal aberrations are rare, and tumors of the distal colon, where microsatellite instability is less frequent and chromosomal aberrations are common. Constitutional chromosomal instability is a hallmark of the Nijmegen breakage syndrome (NBS), a disorder in which the NBS1 gene is mutated. In a previous study, we found an elevated frequency of allelic imbalance (AI) at the 8q21.3 NBS1 locus in proximal, though not distal CRCs. This result suggested that the loss of NBS1 might contribute to the genetic instability, and thus the malignant progression, of proximal CRC. We therefore sequenced the 16 exons of the NBS1 gene in all cases where we had observed AI. Of the 29 cases, none showed any sequence anomaly, although several polymorphisms were found. We also studied markers flanking the recently described Rad54B gene, which is a member of the Rad52 epistasis group to which NBS1 itself belongs. Mutations of this gene, located a few cM distal to NBS1, have been found in colorectal cancer and in primary lymphomas, and it may thus be the target of AI at 8q21. We found that AI at Rad54B was not frequent, and none was observed in cases showing AI at NBS1.