The human CDKN2A locus encodes 2 distinct proteins, p16(INK4A) and p14(ARF) [mouse p19(Arf)], designated INK4A (inhibitor of cyclin dependent kinase 4) and ARF (alternative reading frame) here, that are translated from alternatively spliced mRNAs. Human ARF is implicated as a tumor suppressor gene, mainly in association with the simultaneous deletion of INK4A. However, questions remain as to whether loss of ARF alone is sufficient to drive tumorigenesis. Here, we report that mice deficient for Arf are susceptible to accelerated asbestos-induced malignant mesothelioma (MM). MMs arising in Arf (؉/؊) mice consistently exhibit biallelic inactivation of Arf, but, unexpectedly, do not acquire additional recurrent genetic alterations that we previously identified in asbestos-induced MMs arising in Nf2 (؉/؊) mice. Array CGH analysis was used to detect a recurrent deletion at chromosome 4C6 in MMs from Arf (؉/؊) mice. A candidate gene in this region, Faf1 (FAS-associated factor 1), was further explored, because it encodes a protein implicated in tumor cell survival and in the pathogenesis of some human tumor types. We confirmed hemizygous loss of Faf1 and down-regulation of Faf1 protein in a series of MMs from Arf (؉/؊) mice, and we then showed that Faf1 regulates TNF-␣-mediated NF-B signaling, a pathway previously implicated in asbestos-induced oncogenesis of human mesothelial cells. Collectively, these data indicate that Arf inactivation has a significant role in driving MM pathogenesis, and implicate Faf1 as a key component in the TNF-␣/NF-B signaling node that has now been independently implicated in asbestos-induced oncogenesis.array-CGH ͉ tumor suppressors T he CDKN2A (INK4a/ARF) locus encodes 2 distinct proteins translated from alternatively spliced mRNAs; p16(INK4A), designated as INK4A (inhibitor of cyclin dependent kinase 4) here, is encoded by exons 1␣, 2, and 3. The alternate product p14(ARF), dubbed ARF (alternative reading frame protein) here, is specified by exons 1, 2, and 3 (1, 2). Amino acid sequences of INK4A and ARF are unrelated, because exons 1␣ and 1 show no homology and exon 2/3 sequences are translated in different reading frames.The CDKN2A locus is among the most commonly mutated genomic sites in human cancer (2). Point mutations or deletions specifically affecting exon 1␣ of INK4A are not uncommon, although intragenic mutations affecting exon 1 of ARF are seldom, if ever, observed (2, 3). Overall, the high frequency of concurrent INK4A and ARF loss has made it difficult to assess the contribution of ARF to human tumorigenesis.We previously demonstrated that the CDKN2A locus is homozygously deleted in most malignant mesothelioma (MM) cell lines and in many MM tumor specimens (4). FISH analysis revealed a high incidence (Ϸ50 to 75%) of homozygous deletions in frozen MM specimens or MM cells cultured for Յ5 days (5, 6). Reexpression of INK4A in MM cells resulted in cell cycle arrest, cell death, as well as tumor suppression and regression (7). In contrast to the established role of INK4A, the i...