This stable form of PAI-1 contains an N-terminal cysteine mutation in addition to the four stabilization mutations (patent pending). The protein is supplied in buffer with 1mM DTT to prevent dimerization. To label with thiol reactive dye, buffer exchange into degassed 0.05M Sodium Phosphate; 0.1M NaCl; 1mM EDTA; pH 6.6 by desalting column. Collect into 10 fold molar excess of maleimide or iodoacetamide dye and incubate for 1 hour at room temperature. To preserve PAI-1 activity minimize the percentage of DMSO/DMF in the reaction. Remove free dye by desalting, ammonium sulfate precipitation, or dialysis.