Cytoplasmic domain peptide with an N-terminal signal delivery sequence enters platelets while a peptide lacking the delivery sequence is excluded. Human platelets were incubated with 120 μM FITC-labeled Sig-β₁ peptide or β₁ peptide without the signal delivery sequence for 20 minutes at room temperature. (A) As a positive control for trypan blue quenching, platelets were incubated with the α₂β₁ mAb BHA 2.1 (5 μg/mL) on ice for 10 minutes, followed by an FITC-labeled secondary goat antimouse IgG (10 μg/mL) for 30 minutes, and cell fluorescence was evaluated by flow cytometry before or after addition of trypan blue (2 mg/mL). Data are shown as the normalized mean fluorescence ± SEM from one experiment, representative of 3 similar experiments. (B) Platelet suspensions treated as in panel A were applied to coverslips and visualized by confocal microscopy. Bar represents 10 μm. One of 2 similar experiments is shown.