PMF spectra were acquired by 1600 laser shots (ND:Yag laser, 355 nm) in a mass range between 800 Da and 4500 Da,
 using a 4800 MALDI-TOF/TOF mass spectrometer (Applied Biosystems). Peak lists were generated using a minimum 
signal to noise ratio of 10 and exclusion of prominent human keratin and porcine trypsin autolysis peaks. The 
ten most intense precursor signals were selected for MS/MS fragmentation, performed by 500 laser shots and air 
as collision gas. Peak finding within the MS/MS spectra was performed, using a signal to noise ratio of 10. 
Data were transferred to the GPS Explorer Software V 3.6 (Applied Biosystems) and searched against the 
Mus musculus subset of the Swissprot database (241365 sequences, date 2006/11/23), using the Mascot search engine
 (Matrix Science Ltd., USA) with parameters set as: "Enzyme" = Trypsin; "Fixed modification" = Carbamidomethyl (C); 
"Variable modification" = Oxidation (M); "Max. missed cleavages" = 1; "Precursor tolerance" = 200 ppm; 
"MS/MS fragment tolerance" = 0.5 Da. In the present study, the MS and MS/MS data based Mascot Protein Scores 
had to be above 54 for valid database protein identification results.