Type I interferons (IFN-α/-β) are capable of suppressing c-myc mRNA expression by modulating post-transcriptional processing. However, the molecular mechanism of this phenomenon is poorly understood. We previously established that human polynucleotide phosphorylase (hPNPase old-35), a type I IFN-inducible 3′, 5′ exoribonuclease involved in mRNA degradation, induces G 1 cell cycle arrest and eventually apoptosis by specifically degrading c-myc mRNA. We now demonstrate a close association between IFN-β-induced hPNPase old-35 upregulation and c-myc downregulation in human melanoma cells. Employing stable melanoma cell clones expressing hPNPase old-35 small inhibitory RNA, we demonstrate that hPNPase old-35 is a key molecule coupled with IFN-β-mediated downregulation of c-myc mRNA. Inhibition of hPNPase old-35 or overexpression of c-myc protects melanoma cells from IFN-β-mediated growth inhibition, emphasizing the importance of hPNPase old-35 upregulation and consequent c-myc downregulation in IFN-β-induced growth inhibition and apoptosis induction. In these contexts, targeted overexpression of hPNPase old-35 might be a novel therapeutic strategy for c-myc-overexpressing and IFN-resistant tumors, such as melanomas.