The no-template-DNA control run with each assay ruled out contamination in commercial DNA isolation columns and PCR reagents; in addition, contamination of human DNA samples by FLK DNA was ruled out by testing human DNA with sheep-specific primers (11), and contamination of FLK with other cell lines, including human, was ruled out by using species-specific primers (11).

We thank Gary Firestone for providing the GR cell line; Kathryn Radke for the FLK , [Bat.sub.2][Cl.sub.6], and [Tb.sub.1]Lu cell lines; Carl Hanson for the formalin-fixed HIV- and HTLV-infected cells; Feng Wang-Johanning for the HERV-K DNA; and Craig Meyers for the formalin-fixed keratinocyte raft cultures.