The cell cycle is composed of G1, S, G2 and M phase. The transitions between different phases are regulated at checkpoint such as Start(restriction), S phase and mitotic checkpoint. These checkpoints are regulated by specific cyclins and Cdks(cyclin-dependent kinases). Especially, Start checkpoint in late G1 is though to be very important in control of cell cycle. In this study, it was shown various CDKN2(p16ink4A) alteration, including deletions, mutations, down regulations, and performed differential expression of p53, Cdk4, PCNA and pRb in stomach cancer tissues. 1. The frequency of CDKN2 mutations was not observed in the 19 primary stomach cancer tissues. In contrast to the mutations of CDKN2, mRNA levels was showed by Northern blot analysis that expression of CDKN2 was absent or decreased in 10 of the 19(53%) primary stomach adenocarcinoma. Western blot analysis was performed to determine the differential expression of p53, Cdk4, PCNA related to Start checkpoint. Overexpression of p53 was shown 38%, Cdk4 was expressed in all each specimens, and expression of PCNA was not shown. 2. As the other method to determine the differential expression of p53, Cdk4, PCNA and pRb, immunohistochemical analyses were performed on each 14 formalin-fixed and paraffin embedded tumor tissues of stomach adenocarcinoma. p53 overexpression was showed to clear nuclear staining only in tumor cells not in nonneoplastic cells. In staining for cdk4, the tumor was considered to be cdk4 positive if there was nuclear staining in tumor cells, regardless of cytoplasmic staining. PCNA staining for carcinoma tissues showed more intense nuclear staining in tumor cells than in nonneoplastic cells. pRb overexpression was show in tumor cells. Significant differences were observed in the expression of the proteins among the cancers from different anatomic site. Overexpression of adenocarcinomas had high rate of p53(57.1%) and pRb(71.4% ), and low late of cdk4(7.1% ) and PCNA(14.3% ), As these results, deletion of CDKN2 gene in human stomach cancer was not observed but mRNA expression was down regulated in restriction checkpoint, G1 phase. Inactivation of the CDKN2 gene due to hypermethylation may play an important role in development of cancer. And one of the abnormalities in p53, Cdk4, PCNA or pRb function occurs very common in various cancers, especially oral adenocarcinoma, osteosarcoma and squemous cell carcinoma, suggest that components in restriction checkpoint also play an critical role in the carcinogenesis and progression of cancers.