MANY HUMAN L1 ELEMENTS ARE CAPABLE OF RETROTRANSPOSITION

Using a selective screening strategy to enrich for active L1 elements, we isolated 13 full-length elements from a human genomic library. We tested these and two previously-isolated L1s (L1.3 and L1.4) for rever se transcriptase (RT) activity and the ability to retrotranspose in He La cells, Of the 13 newly-isolated L1s, eight had RT activity and thre e were able to retrotranspose. L1.3 and L1.4 possessed RT activity and retrotransposed at remarkably high frequencies. These studies bring t he number of characterized active human L1 elements to seven. Based on these and other data, we estimate that 30-60 active L1 elements resid e in the average diploid genome.