Loss of IGF2 imprinting in decoy CTCF–expressing cells. (A) ZF-Sss1–expressing breast cancer MCF7 cells. (B) ZF-Sss1–expressing human skin fibroblast HBF1. (C) ZF-EGFP–expressing MCF7. (D) ZF-EGFP–expressing human skin fibroblast WSF7 cells. Allelic expression of IGF2 was determined by polymorphic restriction enzymes AluI (A, C, and D) and ApaI (B). gDNA, genomic DNA as the positive control; mock, empty lentiviral vector that does not contain the decoy constructs; control, untreated cells.