Mass spectroscopic analysis of the coat protein (CP) of Plasmopara halstedii virus (PhV). A Reflector ISD-MALDI-TOF spectrum. The N-terminal sequence of the intact PhV CP was analyzed by Top-Down MALDI-TOF mass spectrometry. The in source decay (ISD) spectrum shows two major ISD fragment ions of the PhV CP with masses of 1409.74 Da and 1765.93 Da, respectively. These ISD fragment ions masses fit to sequences D74-R85 and D74-R88 of the deduced amino acid sequence of the PhV CP, respectively. The corresponding amino acid sequences of the ISD fragment ions were shown. B MS/MS spectrum of the 1409.74 Da ISD-fragment. The N-terminal sequence of PhV CP DYTVQSNSIVQR deduced from the reflector ISD spectrum (Fig. 1A) was confirmed by MS/MS analysis of the 1409.74 Da ISD fragment ion. The observed b- and y-fragment ions are annotated in the spectrum and the corresponding amino acid sequence covered by b- and y-ions were shown.