Human endogenous retroviruses (HERVs) constitute about 8% of the human genome. There are around 4000 pol-containing retroviral integrations in the human genome, which makes it impractical to measure each of them separately. Therefore we developed a set of degenerate real time PCRs to detect major groups bearing sequence similarities to gammaretroviruses, one of the largest groups of human endogenous retrovirus, and betaretroviruses, some of which have integrated into the human genome most recently and which remain the most intact. It was found that, although both gammaretroviral and betaretroviral RNAs were broadly expressed in various healthy tissues including reproductive tissues and brain, a differential expression pattern was observed. My work further revealed that HERVE and HERVW, two gammaretroviral sequences, were ubiquitously and highly expressed in pathologic and normal female reproductive tissues with tissue specific patterns. Expression of HERVE was higher in endometriotic tissue than in normal endometrium. HERVE and HERVW RNAs were higher in normal ovarian tissue than in ovarian cancer. Besides these tissue- and neoplasia-related differences, there were wide differences in HERV expression among individuals. Next, a selective pattern of HERVW upregulation was demonstrated in SK-N-DZ, a neuroblastoma cell line, upon re-oxygenation after a period of hypoxia or with 5-azacytidine, a demethylating agent. Furthermore, broad and high expressions of gammaretrovirus-like transcripts in different brain areas analyzed were identified. The expression levels were variable among different donors. In conclusion a ubiquitous HERV expression was observed in tissues and cell lines, with various patterns. At this stage the data are not sufficient to conclude whether HERV has any physiological or pathological roles in humans. However, their differential expression patterns are compatible with functional roles of HERV in humans.