ASM-deficient NPA lymphoblasts show a defect in plasma membrane repair that is proportional to the extent of cell permeabilization. (A) ASM activity in lysates of lymphoblasts derived from a normal human control (NC) or an NPA human patient. NPA cells lacked detectable ASM activity. The error bar represents SEM. SM, sphingomyelin. (B) FACS quantification of PI staining in lymphoblasts derived from a normal control or NPA patient. When not exposed to SLO, most cells were impermeable to PI. (C) FACS quantification of PI staining in lymphoblasts derived from a normal control (black) or NPA patient (red) after exposure to SLO. NPA lymphoblasts were defective in plasma membrane resealing when compared with normal lymphoblasts. Without Ca²⁺, both groups of cells remained permeabilized (left); with Ca²⁺, control cells resealed more efficiently (right). (B and C) Percentages correspond to resealed (PI negative) cells in the gated region (dashed lines). The results shown in this figure are representative of several independent experiments.