The human genome contains 100,000 fragments of a repetitive element called the LINE-1 element (L1Hs). Approximately 4,000 of these contain the full 6.2 kb sequence. About 5-10% of the human genome is composed of L1Hs DNA. L1Hs is a retrotransposon that causes insertional mutagenesis upon transposition into sensitive genomic loci. Examples of both somatic and presumed germ-line insertional mutagenesis events caused by L1Hs have been reported. Transposition of the human LINE-1 element (L1Hs) proceeds via an RNA intermediate. Regulation of the transcription of L1Hs is, therefore, the first level of the control of L1Hs transposition. Transcription of L1Hs is tightly regulated. No L1Hs mRNA is observed in most human tissue culture cell lines but transcripts are present in teratocarcinoma cells and in many testicular germ cell tumors. We have continued our investigations of the regulation of L1Hs expression. Previously, we demonstrated that the regulatory signals for the appropriate expression of L1Hs are located entirely within the elements' 5' UTR and that sequences within the first 100 base pairs were most important. Recently we identified teratocarcinoma specific and non cell-specific regulatory elements within the region of bp 60-100. A series of mutations were made that indicated an important role for this region in L1Hs expression. Electro-mobility shift assays revealed at least 6 complexes formed by the binding of nuclear proteins to sequences between bp 60-100. One of the bands was formed by the binding of the ubiquitous Oct 1 protein. Other complexes were specific for embryonal carcinoma cells. We have also continued our investigation of a 140 bp L1Hs enhancer that lies between bp 385-525. We have found that this enhancer shares sequence and organization with important regions of the T-cell receptor a enhancer, and the regulatory regions of HIV and JC Virus. We have continued our efforts to investigate the contribution of isochromosome 12p to the expression of L1Hs. Iso 12p is a feature of both testicular germ cell tumors and the Pallister-Killian Syndrome. In so doing we have developed new methods for expression cloning. These methods are also applicable to identifying and cloning drug resistance genes and we have used them successfully in a model system. We have initiated an effort to clone genes that induce resistance to cis-platin, an important chemotherapeutic agent.