Human endogenous retroviruses (HERVs) are a potential source of genetic diversity in the human genome. Although many of these elements have been inactivated over time by the accumulation of deleterious mutations or internal recombination leading to solo-LTR formation, several members of the HERV-K family have been identified that remain nearly intact and probably represent recent integration events. To determine whether HERV-K elements have caused recent changes in the human genome, we have undertaken a study of the level of HERV-K polymorphism that exists in the human population. By using a high-resolution unblotting technique, we analyzed 13 human-specific HERV-K elements in 18 individuals. We found that solo LTRs have formed at five of these loci. These results enable the estimation of HERV solo-LTR formation in the human genome and indicate that these events occur much more frequently than described in inbred mice. Detailed sequence analysis of one provirus shows that solo-LTR formation occurred at least three separate times in recent history. An unoccupied preintegration site also was present at this locus in two individuals, indicating that although the age of this provirus is estimated to be Ϸ1.2 million years, it has not yet become fixed in the human population.A t least 8% of the human genome is made up of endogenous retroviruses and related sequences, which form Ϸ200 distinct groups and subgroups (1-3). Most of these elements represent ancient retroviral infections, as evidenced by their wide distribution in primate species, and no infectious counterparts of human endogenous retroviruses (HERVs) are known to exist today. Many HERV elements have been found to be at identical sites in both Old World monkeys and apes, which diverged Ϸ25 million years ago (4, 5), implying that the virus that gave rise to them existed at least that long ago. As a consequence of their long residence in the genome, most HERVs have acquired numerous mutations in their coding sequences, rendering them incapable of further colonization of the genome by either reinfection or intracellular retrotransposition. More ancient proviruses have more divergent LTRs, which are identical at the time of integration and acquire mutations over time (6). Inactivation of HERV elements can occur also by solo-LTR formation, which is the result of homologous recombination between the two LTRs flanking the provirus and the subsequent deletion of the internal sequence. The vast majority of HERV elements in the human genome exist as solo LTRs (7).The elements in our genome that appear to be the most recently active belong to the HERV-K family. The oldest members of this family entered the genome before the Old World monkey-ape divergence, but HERV-K elements have undergone several periods of expansion throughout primate evolution (7,8). For example, there are many HERV-K elements that are present in the African great apes and humans but not in orangutans and the lesser apes, indicating a relatively recent integration time of 8-15 million years (4, 9)....