The cyclin-dependent kinase inhibitor p16INK4a encoded by the INK4A/CDKN2A/MTS1 gene is a frequent target of 9p21 inactivation in human lung cancers. The p14ARF transcript, which is an alternative spliced form of this locus, is also altered or deleted in a proportion of human lung cancers and has been shown to inhibit cell cycle progression as an endogenous cellular regulator of the p53 protein, raising the possibility that it might constitute an additional lung tumor suppressor gene at the 9p21 locus. To test the candidacy of p14ARF as a lung cancer suppressor and assess the role it plays in radiosensitivity, we transfected the wild-type p14ARF gene into four cell lines which had various endogenous gene backgrounds of INK4A-/p53+/RB+ (A549 and H460), INK4A+/p53+/RB- (H446) as well as p14ARF+/p53-/RB+ (Calu-1). We found that transfection of p14ARF is related to an obvious growth inhibition in all wtp53 cell lines, regardless of INK4A/ARF and RB status. Although it has been shown that p53-induced G1 checkpoint in response to DNA damage by ionizing radiation is p14ARF-independent, we found the radiosensitivity of two p14ARF-deficient cell lines was increased after p14ARF gene transfer. The results indicated that cell cycle redistribution after acquiring the exogenous gene might be the main explanation for the enhanced sensitization. An increased radiation-induced apoptotic proportion in one cell line also suggested a fortified p53 function that might be triggered by the restored p14ARF protein.