The p15 and p16 CDK4 inhibitor genes map within the chromosome band 9p21 region 
deleted frequently in malignant mesothelioma and other cancers. p16 has been 
implicated recently as a potential target of 9p21 deletions in mesothelioma, but 
the role of this gene is uncertain because deletions have been detected more 
often in established cell lines than in primary tumor specimens. We determined 
p15 and p16 copy number by fluorescence in situ hybridization with a P1 contig 
in 50 primary mesotheliomas. Codeletion of p15 and p16 was found in 72% of 
mesotheliomas, including all cases with spindle-cell components (n = 21) and 
total deletion of p15 and p16 was found in several mesotheliomas that lacked 
cytogenetic deletion of the chromosome 9 short arm. Point mutations were not 
found, however, in exon 2 of retained p15 and p16 alleles from seven 
mesotheliomas. These findings demonstrate that p15, p16 and/or a closely 
neighboring gene, are the targets of frequent chromosome 9p deletion in primary 
malignant mesothelioma.