The inhibitor of cyclin-dependent kinase 4-p16INK4 alpha and its alterative reading frame (ARF)-INK4/ARF gene locates at CDKN2A locus of human chromosome 9p21. This locus encodes two overlapped genes: ARF gene and p16INK4 alpha. The amino acid sequences of two genes are completely different because they are encoded by alternative reading frames. ARF participates in the regulation of mdm2-p53 pathway by mdm2. Recent studies showed that ARF gene may play a role in tumorigenesis; the ARF gene promoter hypermethylation may be the principal mechanism in the inactivation of this gene. Here is a review of ARF-mdm2-p53 interacting pathway.