A subset of human immunodeficiency virus type 1 long-term non-progressors is characterized by the unique presence of ancestral sequences in the viral population b 2 m level, and they were also associated with better use of safe-sex practices and higher presence of the HLA sB58 supertype than the modern subset. Viral dating has therefore permitted the segregation of LTNPs into two subsets that show very different virological, immunological, host and clinical-epidemiological characteristics. Moreover, whereas the modern subset displayed low levels of virus replication, the ancestral group displayed not only a very limited virus replication, often to undetectable levels, but also very slow or arrested viral evolution, maintaining the close relationship of the viral population to the transmitted virus. INTRODUCTIONThe natural history of human immunodeficiency virus type 1 (HIV-1) infection is heterogeneous because of the diverse duration of the AIDS-free period, although the median time of appearance of AIDS clinical symptoms is between 8 and 10 years after infection (Bacchetti & Moss, 1989; Hendriks et al., 1993; Muñoz et al., 1989). However, cohort studies have identified a small fraction of infected people, about 5-10 % (Buchbinder et al., 1994;Keet et al., 1994; Lefrère et al., 1997; Sheppard et al., 1993), who, despite infection for more than 10 years, remain symptomless and maintain a relatively high CD4 + cell count (>500 CD4 + cells ml 21 ) without antiviral therapy (Buchbinder et al., 1994;Keet et al., 1994;Learmont et al., 1992;Lifson et al., 1991; Sheppard et al., 1993). Different designations have been used for these patients, such as non-progressors (Lifson et al., 1991; Sheppard et al., 1993), long-term survivors (Levy, 1993), long-term asymptomatics (Keet et al., 1994) and long-term non-progressors (LTNPs) (Buchbinder et al., 1994).LTNPs have been associated with a low viral burden (Barker et al., 1998;Cao et al., 1995;Hogervorst et al., 1995;Pantaleo et al., 1995;Rinaldo et al., 1995) and a strong cellular (Barker et al., 1998;Cao et al., 1995;Harrer et al., 1996;Hogervorst et al., 1995;Klein et al., 1995;Pantaleo et al., 1995;Rinaldo et al., 1995)

HLA-DPB1 can mediate alloreactive responses. A single HLA-DPB1 mismatch increases the risk of aGVHD after sibling donor stem cell transplantation.

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Four new HLA classical class I alleles in the three loci are described in Caucasian individuals. A*3012 was first suspected by an abnormal serologic pattern that would be explained by the single amino acid substitution at the A30-specific Ser17. B*270505 differs from B*270502 in a silent substitution at an up to now constant position in the B locus. B*3541 encodes for a new Cys at position 118 that has not been encountered in neither human nor primate alleles. Cw*0716 seems to be originated by a large-scale interallelic recombination event between Cw*0701/*0706/*0718 and Cw*020202, giving rise to a new antigen-binding cleft conformation.

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