This work is directed toward understanding the mechanism and regulation of transposition of the only transposable element known in the human genome, the class II retrotransposon, LINE-1 (L1Hs). This year, the work has concentrated on the proteins encoded by the two L1Hs open reading frames (ORFs). In contrast to the inconclusive experiments reported last year, we have now obtained clear data indicating that p40, the product of ORF1, occurs in large multimeric ribonucleoprotein complexes in the cytoplasm of human teratocarcinoma cells. The p40 is associated with L1Hs RNA in these complexes. This RNA is available to RNaseA, and after ribonuclease digestion, the complex dissociates to smaller multimers of p40. After several years of work, ORF2, which encodes the L1Hs reverse transcriptase, has been expressed in a heterologous system, namely yeast. Active reverse transcriptase as well as ORF2 protein is detectable in the yeast cell extracts and purification and characterization are under way.