Chromosome 3p abnormalities and allele loss are frequent in lung and breast cancers, and several lung cancer cell lines exhibit homozygous deletions of 3p indicating potential sites of tumor suppressor genes at regions 3p21.3, 3p14.2 and 3p12. We have identified and characterized a new 3p21.3 homozygous deletion in a breast cancer cell line and the primary tumor that overlaps those previously described in small cell lung cancer (SCLC). This homozygous deletion is approximately 220 kb in length and represents a somatically acquired change in the primary breast cancer. Cloning and sequencing of the breakpoint demonstrated that this resulted from an interstitial deletion and precisely pinpoints this deletion within the three SCLC homozygous deletions previously reported. This deletion significantly narrows the minimum common deleted region to 120 kb and is distinct from the previously reported region that suppresses tumor formation of the murine A9 fibrosarcoma cells. These findings suggest that a common homozygous deletion region on 3p21.3 is important in both lung and breast cancers. It is likely that this very well characterized region either contains one tumor suppressor gene common to both tumor types or two closely linked tumor suppressor genes specific for each tumor.