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Schematic drawings of the integrating component of retroviral vector SSR#69 before and after Cre-recombination. SSR#69 contains the hygromycin B resistance gene (Hyg R) as a positive selectable marker and the herpes simplex virus thymidine kinase gene (HSV-TK) as a negative selectable marker. The SV40T, Hyg R and HSV-TK genes are flanked by loxP sites.
