Frequent loss of heterozygosity (LOH) on the short arm of chromosome 1 (1p) has been reported in a series of human malignancies. To investigate the possible existence of tumor suppressor locus (or loci), we examined 41 primary oral squamous cell carcinomas (OSCCs) for LOH using a panel of 15 polymorphic microsatellite markers located on 1p. LOH was observed in 30 of 41 cases (73%) that were informative for at least one of the loci analyzed. Two distinct regions of common allelic loss were identified: a distal region at D1S243 (1p36.3), and proximal region at D1S160 (1p36.1). In addition, the possible involvement of the p73, a candidate tumor suppressor gene located on 1p36.3, was also evaluated. Polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis revealed no mutation of the gene in all samples analyzed (n=41). On the other hand, semi-quantitative reverse transcription-PCR (RT-PCR) demonstrated that 25% of primary tumors (n=20) had absent or reduced mRNA expression of the p73 gene. All cases showing down-regulation of the p73 gene were clinically classified as stage IV, but we could not detect any LOH at the gene locus in the same samples. Furthermore, re-expression of the p73 gene mRNA was induced in OSCC-derived cell lines showing down-regulation of the gene expression after treatment with 5-aza-2'-deoxycytidine, a DNA demethylating agent. These findings suggest that there may be at least two distinct tumor suppressor genes inactivated by allelic deletion on 1p31.1 and 1p31.3, respectively. In addition, the p73 gene could be inactivated by the methylation-dependent silencing of this gene, and associated with the tumor progression of human OSCC.