Expression of either wild-type or R24Q human TFPI-2 by stably transfected HT-1080 cells in the presence and absence of G418. Stably transfected HT-1080 cell lines, with an initial expression level of approximately 55 ng TFPI-2/mL /day/10⁶ cells, was continuously cultured with passaging in the presence (□, ▵) and absence (▪, ▴) of 0.6 mg/mL G418. The supernatants were assayed weekly for either TFPI-2 (□, ▪) or R24Q TFPI-2 (▵, ▴) expression by a sandwich ELISA as described in “Materials and methods.”