The study shows that LINE-1 (L1) retrotransposons, which make up 17% of the human genome, are mostly inactive due to mutations and structural defects. However, a subset of L1 elements, called "hot L1s," are active and capable of retrotransposition. The researchers analyzed 90 intact L1s from the human genome working draft and found that 44% of them are polymorphic, meaning they vary among individuals. Of these, 49% showed retrotransposition activity in cultured cells. They estimate that an average human genome contains 80–100 retrotransposition-competent L1s, with 84% of the activity coming from six highly active "hot L1s." These hot L1s are responsible for most of the retrotransposition in the human population, despite being relatively rare in an individual genome. The study also shows that some L1s involved in recent human insertions are as active as the hot L1s. The researchers conclude that although rare in an individual genome, hot L1s are responsible for the majority of L1 retrotransposition in the human population. The study also highlights the importance of understanding the distribution and activity of L1 elements in the human genome, as they can contribute to genetic variation and disease. The findings have implications for understanding the dynamics of genome evolution and the role of retrotransposons in human genetics.The study shows that LINE-1 (L1) retrotransposons, which make up 17% of the human genome, are mostly inactive due to mutations and structural defects. However, a subset of L1 elements, called "hot L1s," are active and capable of retrotransposition. The researchers analyzed 90 intact L1s from the human genome working draft and found that 44% of them are polymorphic, meaning they vary among individuals. Of these, 49% showed retrotransposition activity in cultured cells. They estimate that an average human genome contains 80–100 retrotransposition-competent L1s, with 84% of the activity coming from six highly active "hot L1s." These hot L1s are responsible for most of the retrotransposition in the human population, despite being relatively rare in an individual genome. The study also shows that some L1s involved in recent human insertions are as active as the hot L1s. The researchers conclude that although rare in an individual genome, hot L1s are responsible for the majority of L1 retrotransposition in the human population. The study also highlights the importance of understanding the distribution and activity of L1 elements in the human genome, as they can contribute to genetic variation and disease. The findings have implications for understanding the dynamics of genome evolution and the role of retrotransposons in human genetics.