Loss of chromosome 3p is a critical event in the pathogenesis of lung cancer. Overlapping homozygous 3p21. 3 deletions in lung cancer cell lines involving GNAI2 were characterized and found to involve a region of genomic instability. A new widely expressed Semaphorin, H. SemaIV, was isolated from the GNAI2 deletion region. Reduced H. SemaIV expression allowed identification of additional cell lines with submicroscopic or larger deletions of the locus which occurred in a heterogeneous manner. We also demonstrate the presence of a distinct 3p21. 3 homozygous deletion region, adjacent to the DNA mismatch repair gene, hMLH1, and identified deletions in direct tumors. This appears to represent one of the first demonstrations of homozygous deletions affecting 3p in direct lung tumors.