The LINE-1 family of interspersed repeated sequences is ubiquitous in mammals. Genetic analysis (by others) has demonstrated that in humans, LINE-1 elements are transposable and mutagenic. Lacking the terminal repeats characteristic of most known classes of prokaryotic and eukaryotic transposable elements, mobility of the LINE-1 family (and related elements in invertebrates) must involve unique mechanisms. This work aims to understand LINE-1 transposition in human cells. Experimental approaches are informed by the following widely-discussed model. One or more of the approximately 4000 full length LINE-1 elements in the human genome are transcribed and translated. Among the translation products is a reverse transcriptase that reverse transcribes LINE-1 RNAs. The resulting cDNAs are inserted into randomly located staggered cleavages in genomic DNA. In support of this model, we have demonstrated that 1) the 900 bp long untranslatable 5' end of some 6 kbp human LINE-1 sequences has promoter activity in human and monkey cells permitting synthesis of a marker gene product (chloramphenicol transacetylase) fused downstream; 2) the 5' ends of a small subset of LINE-1 elements are hypomethylated in NTera2D1 cells (a teratocarcinoma line) the only human cell type in which polyadenylated, cytoplasmic LINE-1 DNA has been observed; 3) anti- serum raised against a synthetic peptide predicted by LINE-1 ORF2 (the ORF that predicts a polypeptide with reverse transcriptase homology) recognizes a trpE/ORF 2 fusion protein made in E. coli and a 108 kDa polypeptide associated with the 100,000 x g pellet of the cytoplasmic fraction of Ntera2D2 cell extracts. Repeated DNA sequence probes are being used to study genome evolution and species identity in New World primates.