ferred from the epidemiological features of the incipient epidemic in western Europe, with most AIDS cases being diagnosed among homosexual men, who frequently referred to traveling to the United States in previous years [1, 2], and from the initial association in this population between HIV-1 seropositivity and recent visits to the United States [3]. Consistent with these epidemiological data, European homosexual men are almost uniformlyinfected with subtype B [4, 5], first introduced among homosexual men in the United States, with phylogenetic trees supporting multiple introductions in western Europe [6]. The subtype B epidemic subsequently spread to injection drug users (IDUs) in western Europe, either from local epidemics among homosexual men or, in some countries, possibly from a variant originating among North American IDUs [4, 5, 7]. In addition, HIV-1 African dclades were also introduced early in the epidemic in some countries by African immigrants or European natives who had traveled to or lived in sub-Saharan Africa and were infected via heterosexual contact, with most of the earliest cases reported in Belgium and France in patients linked to Central Africa [2, 8-10]. In contrast to subtype B, in most western European countries propagation of African dclades has been limited to persons with close epidemiological links to Africa (with some exceptions, discussed below) [11-15]. In the former Soviet Union and in the republics that emerged from its disintegration in 1991, the initial propagation of HIV-1 was much more limited than in western Europe, affecting children in southern Russia (Elista and Rostov-onDon) who were infected during nosocomial outbreaks caused by a subtype G strain in 1988-1989 [16, 17], homosexual men infected with subtype B viruses [1719], and individuals infected via heterosexual contact harboring diverse African clades, mainly immigrants from sub-Saharan Africa or individuals epidemiologically linked to this area [16, 18]. The epidemiological picture of the former Soviet Union epidemic started to change dramatically in 1995 with the occurrence of successive outbreaks among IDUs, first in southern Ukraine [20], then in some areas of Russia [21, 22] and Belarus [23], and subsequently in other areas. The HIV-1 epidemic among IDUs in the former Soviet Union is largely dominated by a subtype A variant of monophyletic origin (variously designated as IDU-A, FSU-A, or AFSU) that is distinct from the major African variants [21, 23-26]; it was first introduced in the Ukrainian city of Odessa [27, 28], on the Black Sea, and has spread widely among IDUs to virtually all former Soviet Union countries, with a more recent secondary spread via heterosexual transmission [26, 29, 30]. Therefore, 2 clades dominate the HIV1 epidemic in Europe, subtype B and the Ansu variant, which, according to their geographical spread, divide Europe in 2 major areas: the subtype B area, extending from western Europe to the borders of the former Soviet Union, and the AFsU area, comprising former Soviet Union countries. However, there are exceptions to this division (figure 1). Thus, in former Soviet Union countries, subtype B is the major clade among homosexual men [25, 26, 29] and predominates in the Ukrainian city of Nikolayev on the Black Sea [31], where a subtype B variant was introduced among