In recent years, MSM have become the most significant increasing HIV-1 transmission route in China. Although several investigations focusing on MSM populations have been conducted, this is the first reported systematic cross-sectional investigation of MSM covering one epidemic region (Anhui province). In accordance with the HIV epidemic in China, Anhui is a severely HIV infected region with more than 10,000 FBD infections that now faces a new challenge in the MSM population.
As an inland and labor force-exporting province, Anhui is a typical accepting epidemic region of HIV infection and transmission. Thus, the investigation of Anhui MSM not only revealed local molecular epidemiology characteristics but also partially characterized HIV infection in all Chinese MSM. The Anhui MSM patients identified may have been infected locally or in other epidemic regions, including Jiangshu, Zhejiang, Shandong, Shanghai, and Tianjing, which may provide epidemiological evidence for the tight linkage of MSM nationwide. Moreover, by analyzing the transmission linkage between Anhui MSM and other MSM, our study first indicated close transmission linkage among the different epidemic regions in Chinese MSM.
As described previously, CRF01_AE, CRF07_BC, and B subtypes are the three major subtypes in the MSM population 
. Estimation of tMRCA for the three subtypes (CRF01_AE, CRF07_BC, B) in Chinese MSM clearly indicated different stages of HIV-1 transmission into MSM. The B subtype, which is the earliest circulating strain (around 1985) in the MSM population, is now declining. Not only that, US-derived B and Thai-B were first observed in Anhui and Henan MSM, which may be attributed to the local number of FBD infections with Thai-B. Notably, Thai-B and US-B subtype strains in MSM showed a closer relationship with MSM-derived B subtype strains from other epidemic regions rather than local circulating B strains in other high-risk populations (such as FBDs). These results clearly indicate a close genetic relationship among MSM strains.
CRF01_AE, entering into MSM in 2000, was the second earliest circulating strain and accounted for more than a half of the infections. CRF07_BC, entering into MSM in 2003, was the third entering strain, causing around 30% of the infection of MSM. As described in , the age of those infected with B subtypes was generally older than those infected with other subtypes, in accordance with the estimated introduction times of B subtypes into Anhui MSM. Combined with the timeline of different subtypes entering into MSM, the differing prevalence of the three subtypes in MSM further suggested some advantage in the newly entering strains that have been overtaking the B subtype in MSM. This was confirmed by observing dynamic changes in the prevalence rate of B, CRF01_AE, and CRF07_BC during 4 years of follow-up in Beijing MSM by Wang et al. 
The MCMC trees of the three subtypes also showed significant transmission linkage among MSM strains from different epidemic regions: MSM strains from different epidemic regions always formed a close clade with high posterior probability. As mentioned above, the depicted epidemic information in and also supported this close transmission linkage among MSM. Active transmission of HIV in Chinese MSM may facilitate the emergence of new recombinant strains. In this study, we identified several recombinant strains among subtype B, subtype C, CRF01_AE, and CRF07_BC in Anhui MSM. Particularly, 4 newly identified 01B recombinant stains, originating in Chinese MSM, showed distinct mosaic models with the isolated 01B strains from foreign countries, and are presumably well-adapted for transmission in the Chinese MSM population. Notably, the active transmission of HIV in MSM undoubtedly facilitates the transmission of emerging 01B recombinant strains in China, which may serve as a reminder of the importance of continuously monitoring the circulating strains in MSM.
As the main lab-force, the MSM are generally young and supposed to be competent in inducing immune response against invading pathogens. However, many patients with age less than 25 in Anhui MSM were diagnosed to be progressive into AIDS. Especially for the new recombinants infections, they all have low CD4 cell counts (<500 cells/µL) and more than 50% had obvious AIDS-like symptoms. This high AIDS progression rate calls for both further expanding investigation and early intervention therapy for the infected MSM.
Taken together, our studies not only characterized the virology and demographic characteristics of the Anhui MSM population, but also systematically revealed active transmission across the Chinese MSM population. The potential advantage and enhanced pathogenesis of emerging circulating recombinant strains, plus active transmission in MSM, make a new HIV pandemic possible in the Chinese MSM population.