Reduced expression and partial loss of function of A91V and the coinherited substitution R232S. The effect of PRF1 mutations identified in fraternal twins inheriting A91V, R232H, and doubly mutated A91V/R232H perforin (PFP).²¹  The top panel shows a Western blot of whole-cell extracts from RBL cells expressing the respective mutated perforin and sorted as described in “Materials and methods.” The graphs show 4-hour cytotoxicity assays using transiently transfected and sorted RBL cells as effector cells and ⁵¹Cr-labeled Jurkat cells as targets at the effector-target (E/T) ratios indicated. The data shown are the means ± SE of 4 to 9 independent experiments. For clarity, a subset of the data (the lower E/T ratios) is shown again in the larger plot.