RNase protection analysis of constitutive ISG expression in P2.1 cells. (A) Analysis of RNA from untreated 2C4 cells (lane 1), IFNα-treated 2C4 cells (lane 2), untreated U4C cells (lane 3), and untreated P2.1 cells (lane 4). Total cellular RNA (10 μg) was analyzed with probes for p48, IRF-1, 9–27, and γ-actin. (B) Analysis of the dominance of the constitutive phenotype. Puromycin-resistant P2.1 cells were fused with hygromycin-resistant U4A cells, and heterokaryons were selected with both drugs. FACS analysis of CD2 expression in U4C cells (Top), P2.1 cells (Middle), and P2.1/U4A heterokaryons (Bottom).