FlG. 1. Construction of retrovirus-HPRT hybrid plasmids Recombinant plasmids were made using standard techniques (26, 27). Restriction 
enzyme abbreviations are P, Pst I; B, SamHI; H, tftndlll; E, EcoRl; R, Rsa I; Hp, Hpa I; S, Sal I. To reduce unwanted ligation products, the indicated 
fragments were treated with bovine alkaline phosphatase (BAP). Cross-hatched segments represent HPRT cDNA, boxes with interior arrows rep- 
resent viral LTRs with orientation indicated by the arrow (5' — » 3"), thin lines indicate viral sequences, thick lines indicate pBR322, and wavy lines 
indicate mouse cell sequences surrounding proviral DNA. 
i 
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