In contrast to most other human endogenous retroviral families, various HERV-K members have open reading frames that code for functional viral proteins which can form noninfectious particles in some germ cell tumors. The HERV-K viral genes are highly transcribed in germ cell tumors but are transcribed to lower or undetectable levels in most other tissue and tumor types. To further analyze the expression patterns of these proviruses, long terminal repeats (LTRs) were isolated from the human genome and used in reporter gene assays. Expression of some HERV-K LTRs was found to be high in human and murine germ cell tumors (testicular teratocarcinomas) and low in non-germ-cell tumors. Furthermore, upon differentiation of a teratocarcinoma cell line, the expression of an active LTR dropped dramatically, suggesting developmental regulation of these proviral LTRs. Transgenic mice harboring an active LTR driving lacZ expression were generated and analyzed. Adult mouse testes showed the highest levels of expression, and the transgene staining appeared to be restricted primarily to the more undifferentiated spermatocytes. Most other tissues analyzed revealed very low or undetectable levels of expression both by reverse transcription-PCR and by Northern blot analysis. Whether the restricted expression of HERV-K in germ cells and in germ cell-derived tumors is of significant importance during development or tumorigenesis remains to be elucidated. Germ line expression of these viruses would allow for their expansion and movement, while somatic repression would ensure limited insertional mutagenesis and misexpression in an individual.