Southern blot analysis of mice containing targeted deletions of 5′ HS 5 and 6. DNA was isolated from mouse tails and digested with HpaI (H), blotted, and hybridized to an upstream probe (KH end). Lane 1: wild-type (WT) mouse; lane 2: ▵5,6 H/WT mouse; lane 3: ▵5,6 H/▵5,6 H mouse; lane 4: ▵5,6 ▵H/WT mouse; lane 5: ▵5,6 ▵H/▵5,6 ▵H mouse; M: molecular size standards. The targeted mutation replaced a 3.5-kb Hpa I to EcoRV fragment with a 2.0-kb PGK-hygro marker, destroying both restriction enzyme sites and leading to a larger Hpa I restriction fragment (▵5,6 H; lanes 2 and 3). Excision of PGK-hygro decreases the restriction fragment size by 2.0 kb (▵5,6 ▵H; lanes 4 and 5). Triangles represent lox P sites.