p16INK4A, p15INK4B, and p18 proteins are highly specific inhibitors of cyclin-dependent serine/threonine kinase (CDK) activities required for GI-S transition in the eukaryotic cell division cycle. Mutations, mainly homozygous deletions, of the CDKN2A (p16INK4A/MTSI) gene have been recently found in tumor cell lines and in many primary tumors. We looked for homozygous deletions of CDKN2A, CDKN2B (p15INK4B), and CDKN2C (p18) in 12 primary rhabdomyosarcoma (RMS) specimens and in five cell lines established from this cancer type. By means of polymerase chain reaction (PCR) and PCR-single strand conformation polymorphism (PCR-SSCP), we analyzed the presence of biallelic gene deletion or point mutation causing gene function loss. All the examined tumor cell lines (100%) and three of 12 (25%) primary tumors showed homozygous deletion of CDKN2A. Furthermore, no aberrant bands in primary tumors were detected via SSCP, suggesting the absence of mutations in the coding region. In all cases the deleted area at 9p21 also involved the CDKN2B gene. Conversely, no homozygous deletion or point mutations were detected when CDKN2C was analyzed. Our results strongly indicate that the p16INK4A (and/or p15INK4B) protein plays a key role in the development and/or progression of childhood rhabdomyosarcoma and suggest that this CDK-inhibitor protein might control proliferation and/or differentiation of human muscle cells. Moreover, alteration of CDKN2C does not appear to be involved in the genesis of rhabdomyosarcoma.