A molecular epidemiology study was conducted among more than 100 human immunodeficiency computer virus type 1 (HIV-1) subtype C seropositive intravenous drug users (IDUs) from China. all subtype C strains isolated from IDUs living in different areas of China, suggesting a common ancestor for the SR-13668 supplier C/B recombinant strains. More than 50% of well-defined subtype B-derived cytotoxic T-lymphocyte epitopes within Gag and Pol and 10% of the known epitopes in Env were found to exactly match sequences within in this clade C/B chimeric reference strain. These results may substantially facilitate a biological comparison of clade C-derived reference strains as well as the generation of useful reagents supporting vaccine-related efforts in China. Human immunodeficiency computer virus (HIV) evolves by the rapid accumulation of mutations and intersubtype recombinations. Different subtypes cocirculating in the population of a geographical region represent the molecular basis for the generation and distribution of interclade mosaic viruses. Although the global HIV-1 variants have been studied intensively by means of serologic testing and heteroduplex DNA analysis, most phylogenetic studies are based on envelope sequences. Many of the prevalent subtypes and a variety of recombinant forms lack fully sequenced genomes. The increasing number of full-length HIV-1 genomes published recently in the databases indicate that full-length viral sequences are necessary for an optimal characterization of the phylogenetic relationship between a new SR-13668 supplier isolate and the pre-existing HIV sequences, particularly in light of the potential for recombination (3, 4, 11, 12). A good example is provided by clade E viruses, which caused the major epidemics in Southeast Asia. Initially these viruses were classified as subtype E solely on the basis of envelope genotyping. Later they were shown to be members of an A/E recombinant strain by full-length genome sequences analysis (4, 12). Each HIV epidemic in distinct geographical regions and populace groups has its own specific characteristics and dynamics. In Asia, the HIV epidemic has spread extensively since the 1980s, with multiple, genetically divergent subtypes (38), complicating the development of effective vaccines for the affected countries (7, 8, 16). The experience in Thailand illustrates the potential for rapid HIV transmission in this area. Yunnan, SR-13668 supplier a southwestern province of China bordering the drug triangle of Myanmar, Laos, and Thailand, was identified in the late 1980s as the first epidemic region in China, with prototype B strains circulating throughout the group of intravenous drug users (IDUs) (31, SR-13668 supplier 42; Y. Ma, Z. Li, K. Zhang, et al., Abstract, Chin. J. Epidemiol. 11:184, 1990). With time a shift occurred toward B-Thai (B) genotypes, and the former predominant prototype B has now been taken over by B-Thai variants (15, 36). The second epidemic was imported to the same area in the early 1990s, most probably by Indian IDUs carrying subtype C strains (30; C. C. Luo, C. Tian, D. J. Hu, M. Kai, T. Dondero, and X. Zhang, Letter, Lancet 345:1051C1052, 1995). Within a few years, subtype C viruses spread rapidly in southern, central, and even in northwestern China by drug trafficking and caused a widespread epidemic in China. According to a recent Chinese nationwide HIV molecular epidemiology survey, almost all the individuals infected with subtype C are IDUs and they include about 40% of HIV-infected IDUs in China, suggesting that subtype C is one of the major HIV-1 subtypes prevalent among IDUs in China (32; Y. Shao, L. Su, X. H. Sun, et Bnip3 al., Abstr. 12th World AIDS Conf., abstr. 13132, 1998). This suggests that the HIV epidemic among SR-13668 supplier IDUs in China extended from a single predominant subtype (B) within a few years.