Loss of heterozygosity at the polymorphic loci on the long arm of chromosome 5 is observed in about 80% of human small-cell lung cancer (SCLC). Absence of inactivating mutations in the APC gene on 5q14 suggests the involvement of another tumor suppressor gene. We found a homozygous deletion of sequence tagged site sequence G73332 on 5q12.3 in 2 of 12 human SCLC cell lines, Lu130 and Lu134. One copy of chromosome 5q was lost in these cell lines, and the remaining allele had a deletion in a more restricted region. A polymerase chain reaction-based analysis of yeast artificial chromosome, bacterial artificial chromosome (BAC), and lambda-phage clones narrowed the region of homozygous deletion to a fragment cloned within one BAC. Sequencing analysis revealed that a DNA fragment of approximately 25kb was deleted interstitially, probably because of recombination through Alu repetitive sequences in Lu130 and Lu134 cells. This deletion was not detected in normal lymphocyte DNA from 98 unrelated individuals. No candidate genes, however, were detected within this region or in the adjacent 150-kb fragment. The absence of microsatellite instability and the presence of an interstitial deletion as well as gross chromosomal aberration suggest that the genomic integrity of Lu130 and Lu134 cells might possibly be affected by Alu-mediated recombination in addition to chromosomal instability. The identical breakpoints in Lu134 and Lu135 cells as well as the same genotypes at all 33 polymorphic loci examined on various chromosomes strongly suggest that these cell lines share the same genetic materials, at least in part, during the establishment or propagation of cell lines.