Samples of lyophilized His 6 -proHNP1A (150 mg) or His 6 -proHNP1B (550 mg) were dissolved in 2 ml 80% formic acid containing 20 mg CNBr. The reaction mixtures were sparged with N 2 and incubated for 18 h in the dark at room temperature after which the solutions were diluted 10-fold with water and lyophilized. The reaction products were dissolved in 0.1% acetic acid and purified by RP-HPLC on an analytical C18 column and analyzed by MALDI-TOF MS. Fractions containing HNP1 were purified further by C18 RP-HPLC.