tokens: Cells were fixed for 30 min at room temperature with 4 % paraformaldehyde then permeabilized with 0 . 2 % Triton and blocked with 1 . 5 % normal goat serum , 0 . 1 % bovine serum albumin ( BSA ) in 0 . 1 M phosphate - buffer saline for 1 h to reduce nonspecific binding .