We have previously studied genomic copy number changes and global gene expression patterns in rat osteosarcomas (OS) induced by the bone-seeking alpha emitter (238)Pu by comparative genomic hybridization (CGH) and oligonucleotide microarray analyses, respectively. Among the previously identified genes that were down-regulated in radiation-induced rat OS tumors, Cited2 (Cbp/p300-interacting transactivator, with Glu/Asp-rich carboxy-terminal domain, 2) and Akap12 (a kinase anchoring protein, also known as src-suppressed C-kinase substrate, SSeCKS) genes mapped to the most frequently lost regions on chromosome 1p. In the present study, relative copy number losses of Cited2 and Akap12 genes were observed in 8 of 15 (53%) and 10 of 15 (67%) tumors by quantitative PCR analysis. Loss of Cited2 and Akap12 in the tumors was confirmed at the levels of mRNA and protein expression by quantitative RT-PCR and immunoblot analyses, respectively. These results indicate that Cited2 and Akap12 are silenced in radiation-induced OS, and therefore are novel candidate tumor-suppressor genes of this tumor.