The study by Brouha et al. investigates the activity and distribution of LINE-1 (L1) retrotransposons in the human genome. Despite comprising 17% of the human genome, only a small fraction of L1s are active. The researchers identified 90 intact L1s in the December 2001 human genome draft sequence and found that 49% of these were active in a cultured cell retrotransposition assay. They estimated that there are 80-100 retrotransposition-competent L1s in an average human being. Notably, 84% of the total retrotransposition capability was attributed to six highly active "hot" L1s. The study also found that most de novo human insertions involve hot L1s, suggesting that while hot L1s are rare in individual genomes, they are responsible for the majority of retrotransposition in the human population. The findings provide insights into the dynamics and impact of retrotransposons on human genome evolution.The study by Brouha et al. investigates the activity and distribution of LINE-1 (L1) retrotransposons in the human genome. Despite comprising 17% of the human genome, only a small fraction of L1s are active. The researchers identified 90 intact L1s in the December 2001 human genome draft sequence and found that 49% of these were active in a cultured cell retrotransposition assay. They estimated that there are 80-100 retrotransposition-competent L1s in an average human being. Notably, 84% of the total retrotransposition capability was attributed to six highly active "hot" L1s. The study also found that most de novo human insertions involve hot L1s, suggesting that while hot L1s are rare in individual genomes, they are responsible for the majority of retrotransposition in the human population. The findings provide insights into the dynamics and impact of retrotransposons on human genome evolution.