GC-box-driven luciferase reporter gene activity. A GC-box-luciferase construct was transfected by electroporation into parental CCRF-CEM wild-type (WT) cells (A, C) and their antifolate-resistant subline EDX^R0.03 (B, D), after which a portion of cells was treated with the various agents for 3 hours: IBMX, pCPT-cGMP, pCPT-cGMP plus okadaic acid or okadaic acid alone (A, B), as well as pCPT-cGMP, SNP, or zaprinast (C, D). Cells were then lysed, and reporter gene activities were determined as detailed in “Materials and methods.” Results presented are normalized mean reporter activities (relative to untreated parental cells) ± SD obtained from 3 independent experiments.