Reporter vector constructs. Control reporter vector MGPN2 (22), based on the oncoretrovirus vector murine stem cell virus (6), has two reporter cassettes: GFP transcribed from the virus 5′ LTR promoter, and Neo transcribed from a Pgk promoter. A 1.2-kb fragment containing the cHS4 element (16) was inserted in either orientation into the 3′ LTR to generate vectors INS4(+) and INS4(−). In this location, the cHS4 fragment is copied into the 5′LTR to generate a flanking double-copy (23), as indicated by the dashed lines. The core DNase-I hypersensitive site for the cHS4 fragment is indicated as a solid block. Enh, enhancer; pA, polyadenylation site.