The βc chain promoter has cell-type–specific activity. The 2.7-kb HinDIII fragment was cloned into pBLCAT3 in both forward (F) and reverse (R) orientation and subsequently transfected into U937, HL-60, COS-1, and HeLa cells. Empty vector (V) was used as a negative control; a CAT reporter plasmid driven by an RSV promoter was used as a positive control (RSV). Promoter activity can only be detected in U937, HL-60, and COS-1 cells transfected with the forward construct, indicating that the 2.7-kb HinDIII fragment contains a functional promoter. Values are the mean of at least four independent experiments; the error bars indicate the standard deviation. LacZ determination was used to correct for transfection efficiency.