Endogenous D-type (HERV-K) related sequences are packaged into retroviral particles in the placenta and possess open reading frames for reverse transcriptase.

All primates studied to date produce retroviral-like particles in their placentae. We have purified these particles from two primate species, one Old World (human) and one New World (marmoset), and have identified the retroviral sequences which are packaged into these particles. Three families of sequences have been detected in these particles in human, all of which have the highest homology to B- and D-type retroviruses and to the human endogenous retrovirus HERV-K10. Previous studies have reported that the New World monkeys do not possess sequences with homology to HERV-K10. We have identified a new family of low-copy-number sequences which are present in New World monkeys and which possess 70% homology to the HERV-K family. Particles from both species possess reverse transcriptase activity and we have found that some of these retroviral particles package sequences which encode long open reading frames in pol, as revealed by expression cloning in Escherichia coli. These open reading frames could encode the reverse transcriptase enzyme activity found in the particles.