A cluster of nine retroposons of four different types in a 6221 base EcoRI DNA fragment was isolated from a human fetal liver genomic library using a human nucleophosmin (B23) cDNA as a probe. These retroposons are: (1) a solitary HERV-K long terminal repeat upstream from; (2) a nucleophosmin processed pseudogene; (3) six Alu repeated sequences interspersed in both directions; and (4) a truncated Kpn repeated sequence integrated by an Alu monomer and the HERV-K long terminal repeat. Sequence analysis shows that the nucleophosmin pseudogene contains a long stretch (135 base-pairs) of homopurine.homopyrimidine (Pur.Pyr) sequence. S1 and P1 nuclease digestion indicated that this sequence was able to adopt a non-B-DNA triplex structure under either acidic or neutral conditions. This finding is the first example of the association of a potential DNA triplex structure with a cluster of retroposons.