The mouse p19 Arf (human p14 ARF ) tumor suppressor protein, encoded in part from an alternative reading frame of the Ink4a (Cdkn2a) gene, inhibits the Mdm2 E3 ubiquitin ligase to activate p53. Arf is not expressed in most normal tissues of young mice but is induced by high thresholds of aberrant hyperproliferative signals, thereby activating p53 in incipient tumor cells that have experienced oncogene activation. The single Arf mRNA encodes two distinct polypeptides, including full-length p19 Arf and N-terminally truncated and unstable p15 smArf ("small mitochondrial Arf") initiated from an internal in-frame AUG codon specifying methionine-45. Interactions of p19 Arf with Mdm2, or separately with nucleophosmin (NPM, B23) that localizes and stabilizes p19 Arf within the nucleolus, require p19 Arf N-terminal amino acids that are not present within p15 smArf . We have generated mice that produce either smARF alone or M45A-mutated (smArf-deficient) full-length p19 Arf proteins. BCR-ABL-expressing pro/pre-B cells producing smArf alone are as oncogenic as their Arf-null counterparts in generating acute lymphoblastic leukemia when infused into unconditioned syngeneic mice. In contrast, smArf-deficient cells from mice of the Arf M45A strain are as resistant as wild-type Arf +/+ cells to comparable oncogenic challenge and do not produce tumors. Apart from being prone to tumor development, Arf-null mice are blind, and their male germ cells exhibit defects in meiotic maturation and sperm production. Although Arf M45A mice manifest the latter defects, smArf alone remarkably rescues both of these p53-independent developmental phenotypes.Arf tumor suppressor | p53 | BCR-ABL acute lymphoblastic leukemia | spermatogenesis | hyaloid vasculature T hree exons (1α, 2, and 3) of the Cdkn2a (Ink4a) gene encode spliced mRNA segments that specify the Cdk4/6 inhibitor p16 Ink4a , whereas mRNA initiated from a distinct upstream 5′ exon (1β) opens an alternative reading in Ink4a exons 2 and 3 to yield p19 Arf (1), an inhibitor of the p53 ubiquitin ligase Mdm2 (2-5). As such, the unprecedented structure of the compact Ink4a-Arf locus, conserved in mammals, specifies two distinct tumor suppressors that interface with both the retinoblastoma protein (RB) and p53. The separate promoters upstream of exon-1α (encoding Ink4a) and exon-1β (encoding Arf) are induced by hyperproliferative signals generated by activated oncoproteins, triggering RB-and p53-dependent programs, respectively, that counter cellular self-renewal in response to oncogene stress (6, 7). Deletion or epigenetic silencing of the Ink4a-Arf locus coordinately diminishes the tumor suppressing effects of both RB and p53 and, as would be expected, inactivation of this gene cluster is one of the most frequently observed events in human cancer.The stability and nucleolar sequestration of p19 Arf depend on its interaction with nucleophosmin (NPM or B23), and Arf's ability to bind to either Mdm2 or NPM is determined by N-terminal amino acid residues encoded solely by exon-1β (8-11). Ind...